Disable iOS ScenariosTests suite

This leaves ScenariosUITests enabled.

This also reverts commit 55d447a where
one test was previously disabled. However, another started failing soon
after, so instead we disable the whole suite here.

Author: cbracken

testing/scenario_app/ios/Scenarios/Scenarios.xcodeproj/xcshareddata/xcschemes/Scenarios.xcscheme

@@ -39,7 +39,7 @@
       </MacroExpansion>
       <Testables>
          <TestableReference
-            skipped = "NO"
+            skipped = "YES"
             testExecutionOrdering = "random">
             <BuildableReference
                BuildableIdentifier = "primary"

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

@@ -227,9 +227,7 @@ - (void)testVisibleFlutterViewControllerRespondsToApplicationLifecycle {
   [engine setViewController:nil];
 }
 
-// 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 {
+- (void)testFlutterViewControllerDetachingSendsApplicationLifecycle {
   XCTestExpectation* engineStartedExpectation = [self expectationWithDescription:@"Engine started"];
 
   // Let the engine finish booting (at the end of which the channels are properly set-up) before