fix type error in VS Code

This was passing Oxlint/tsgo check, but showing red squiggle in VS Code (probably older tsc treats it slightly differently).

Author: overlookmotel

apps/oxlint/conformance/src/rule_tester.ts

@@ -105,7 +105,7 @@ function addBeforeHook<T extends TestCase>(test: T): T {
 
   if (Object.hasOwn(test, "before")) {
     // oxlint-disable-next-line typescript/unbound-method - called with `.call`
-    const originalBefore = test.before;
+    const originalBefore = test.before as () => void;
     test.before = function (this) {
       setCurrentTest(clonedTest);
       originalBefore!.call(this);