Disable iOS application lifetime Scenario tests (#21125)

This speculatively disables the ApplicationLifecycle Scenarios iOS tests
which continue to fail after being re-enabled in #21087.

Related P0 issue: flutter/flutter#61620

Author: cbracken

testing/scenario_app/ios/Scenarios/ScenariosTests/AppLifecycleTests.m

@@ -40,7 +40,9 @@ - (void)setUp {
   self.continueAfterFailure = NO;
 }
 
-- (void)testDismissedFlutterViewControllerNotRespondingToApplicationLifecycle {
+// TODO(cbracken): re-enable this test by removing the skip_ prefix once the source of its flakiness
+// has been identified. https://github.com/flutter/flutter/issues/61620
+- (void)skip_testDismissedFlutterViewControllerNotRespondingToApplicationLifecycle {
   XCTestExpectation* engineStartedExpectation = [self expectationWithDescription:@"Engine started"];
 
   // Let the engine finish booting (at the end of which the channels are properly set-up) before
@@ -175,7 +177,9 @@ - (void)testDismissedFlutterViewControllerNotRespondingToApplicationLifecycle {
   [engine destroyContext];
 }
 
-- (void)testVisibleFlutterViewControllerRespondsToApplicationLifecycle {
+// TODO(cbracken): re-enable this test by removing the skip_ prefix once the source of its flakiness
+// has been identified. https://github.com/flutter/flutter/issues/61620
+- (void)skip_testVisibleFlutterViewControllerRespondsToApplicationLifecycle {
   XCTestExpectation* engineStartedExpectation = [self expectationWithDescription:@"Engine started"];
 
   // Let the engine finish booting (at the end of which the channels are properly set-up) before
@@ -280,7 +284,9 @@ - (void)testVisibleFlutterViewControllerRespondsToApplicationLifecycle {
   [engine destroyContext];
 }
 
-- (void)testFlutterViewControllerDetachingSendsApplicationLifecycle {
+// TODO(cbracken): re-enable this test by removing the skip_ prefix once the source of its flakiness
+// has been identified. https://github.com/flutter/flutter/issues/61620
+- (void)skip_testFlutterViewControllerDetachingSendsApplicationLifecycle {
   XCTestExpectation* engineStartedExpectation = [self expectationWithDescription:@"Engine started"];
 
   // Let the engine finish booting (at the end of which the channels are properly set-up) before