[pigeon]enable treat warning as errors for swift code in unit test (#3889)

This will make it easier to spot issues where we accidentally create an "implicit optional". For more details about why implicit optional is bad, see #3658

For example, the following warning was ignored previously: 

![Screenshot 2023-04-06 at 4 15 07 PM](https://user-images.githubusercontent.com/41930132/235790274-776f096a-212c-445b-ae81-f748b143f982.png)

*List which issues are fixed by this PR. You must list at least one issue.*

Mentioned in PR: #3658 (comment)

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

Author: hellohuanlin

packages/pigeon/platform_tests/test_plugin/example/ios/Runner.xcodeproj/project.pbxproj

@@ -542,6 +542,7 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};
@@ -557,6 +558,7 @@
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.plugins.RunnerTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};
@@ -572,6 +574,7 @@
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.plugins.RunnerTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};