Remove duplicate padding on CupertinoFormSection (flutter#137039)

Fixes flutter#128419, removes the duplicate padding that exists around the header and footer on `CupertinoFormSection`. This PR takes the implementation in flutter#129065 and adds tests. I added the ones suggested in the PR review, as well as one more to test that `margin` is passed correctly from `CupertinoFormSection` to `CupertinoListSection`.

Note: I can't quite figure out if this is also a fix to flutter#121543.

Potential review questions:

- Is the use of `offsetMoreOrLessEquals` here correct? I used it because of the vertical positioning. The horizontal positioning is always exact.

### Screenshots
<details>
  <summary>Before</summary>
  
![image](https://github.com/flutter/flutter/assets/5138348/1bce10c9-706a-40c8-a581-2dcb574ed937)
</details>
<details>
  <summary>After</summary>
  
![image](https://github.com/flutter/flutter/assets/5138348/cd1d529b-7be5-45df-af31-0f7760dc3fe9)
</details>

Author: JaspervanRiet

packages/flutter/lib/src/cupertino/form_section.dart

@@ -11,12 +11,6 @@ import 'list_section.dart';
 // iOS 14.2 SDK.
 const EdgeInsetsDirectional _kFormDefaultInsetGroupedRowsMargin = EdgeInsetsDirectional.fromSTEB(20.0, 0.0, 20.0, 10.0);
 
-// Standard header margin, determined from SwiftUI's Forms in iOS 14.2 SDK.
-const EdgeInsetsDirectional _kFormDefaultHeaderMargin = EdgeInsetsDirectional.fromSTEB(20.0, 16.0, 20.0, 10.0);
-
-// Standard footer margin, determined from SwiftUI's Forms in iOS 14.2 SDK.
-const EdgeInsetsDirectional _kFormDefaultFooterMargin = EdgeInsetsDirectional.fromSTEB(20.0, 0.0, 20.0, 10.0);
-
 /// An iOS-style form section.
 ///
 /// The base constructor for [CupertinoFormSection] constructs an
@@ -205,10 +199,7 @@ class CupertinoFormSection extends StatelessWidget {
               fontSize: 13.0,
               color: CupertinoColors.secondaryLabel.resolveFrom(context),
             ),
-            child: Padding(
-              padding: _kFormDefaultHeaderMargin,
-              child: header,
-            ));
+            child: header!);
 
     final Widget? footerWidget = footer == null
         ? null
@@ -217,10 +208,7 @@ class CupertinoFormSection extends StatelessWidget {
               fontSize: 13.0,
               color: CupertinoColors.secondaryLabel.resolveFrom(context),
             ),
-            child: Padding(
-              padding: _kFormDefaultFooterMargin,
-              child: footer,
-            ));
+            child: footer!);
 
     return _type == CupertinoListSectionType.base
         ? CupertinoListSection(

packages/flutter/test/cupertino/form_section_test.dart

@@ -168,4 +168,57 @@ void main() {
     final Iterable<RenderClipRRect> renderClips = tester.allRenderObjects.whereType<RenderClipRRect>();
     expect(renderClips, isEmpty);
   });
+
+  testWidgetsWithLeakTracking('Does not double up padding on header', (WidgetTester tester) async {
+    const Widget header = Text('Header');
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoFormSection(
+            header: header,
+            children: <Widget>[CupertinoTextFormFieldRow()],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getTopLeft(find.byWidget(header)), const Offset(20, 22));
+  });
+
+  testWidgetsWithLeakTracking('Does not double up padding on footer', (WidgetTester tester) async {
+    const Widget footer = Text('Footer');
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoFormSection(
+            footer: footer,
+            children: <Widget>[CupertinoTextFormFieldRow()],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getTopLeft(find.byWidget(footer)), offsetMoreOrLessEquals(const Offset(20, 65), epsilon: 1));
+  });
+
+  testWidgetsWithLeakTracking('Sets custom margin', (WidgetTester tester) async {
+    final Widget child = CupertinoTextFormFieldRow();
+
+    const double margin = 35;
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoFormSection(
+            margin: const EdgeInsets.all(margin),
+            children: <Widget>[child],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getTopLeft(find.byWidget(child)), offsetMoreOrLessEquals(const Offset(margin, 22 + margin), epsilon: 1));
+  });
 }

packages/flutter/test/cupertino/list_section_test.dart

@@ -223,4 +223,44 @@ void main() {
       }
     }
   });
+
+  testWidgetsWithLeakTracking('does not show margin by default', (WidgetTester tester) async {
+    const Widget child = CupertinoListTile(title: Text('CupertinoListTile'));
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoListSection(
+            header: const Text('Header'),
+            children: const <Widget>[
+              child,
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getTopLeft(find.byWidget(child)), offsetMoreOrLessEquals(const Offset(0, 41), epsilon: 1));
+  });
+
+  testWidgetsWithLeakTracking('shows custom margin', (WidgetTester tester) async {
+    const Widget child = CupertinoListTile(title: Text('CupertinoListTile'));
+    const double margin = 10;
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoListSection(
+            header: const Text('Header'),
+            margin: const EdgeInsets.all(margin),
+            children: const <Widget>[
+              child,
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getTopLeft(find.byWidget(child)), offsetMoreOrLessEquals(const Offset(margin, 41 + margin), epsilon: 1));
+  });
 }