Re-enable testKeyWindow for Xcode 12.

GoogleSignIn/Tests/Unit/GIDEMMErrorHandlerTest.m

@@ -476,9 +476,6 @@ NS_ASSUME_NONNULL_BEGIN
   [self testScreenlockRequiredCancel];
 }
 
-// Temporarily disable testKeyWindow for Xcode 12 and under due to unexplained failure.
-#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 150000
-
 // Verifies that the `keyWindow` internal method works on all OS versions as expected.
 - (void)testKeyWindow {
   // The original method has been swizzled in `setUp` so get its original implementation to test.
@@ -524,8 +521,6 @@ NS_ASSUME_NONNULL_BEGIN
               isClassSelector:YES];
 }
 
-#endif  // __IPHONE_OS_VERSION_MAX_ALLOWED >= 150000
-
 @end
 
 NS_ASSUME_NONNULL_END