[macOS] Bail out of tests if engine not running (#47771)

In engine tests where we require a running engine to proceed with the
test, immediately fail if the engine isn't running rather than carrying
on with the test. In most cases, the fixture test proceeds to block the
main thread on a latch until it's released from the UI thread, so this
results in quicker test failures.

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Author: cbracken

shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -123,7 +123,7 @@ + (void)registerWithRegistrar:(id<FlutterPluginRegistrar>)registrar {
 TEST_F(FlutterEngineTest, CanLaunch) {
   FlutterEngine* engine = GetFlutterEngine();
   EXPECT_TRUE([engine runWithEntrypoint:@"main"]);
-  EXPECT_TRUE(engine.running);
+  ASSERT_TRUE(engine.running);
 }
 
 TEST_F(FlutterEngineTest, HasNonNullExecutableName) {
@@ -195,7 +195,7 @@ + (void)registerWithRegistrar:(id<FlutterPluginRegistrar>)registrar {
   // Launch the test entrypoint.
   FlutterEngine* engine = GetFlutterEngine();
   EXPECT_TRUE([engine runWithEntrypoint:@"canLogToStdout"]);
-  EXPECT_TRUE(engine.running);
+  ASSERT_TRUE(engine.running);
 
   latch.Wait();
 
@@ -224,7 +224,7 @@ + (void)registerWithRegistrar:(id<FlutterPluginRegistrar>)registrar {
 
   // Launch the test entrypoint.
   EXPECT_TRUE([engine runWithEntrypoint:@"backgroundTest"]);
-  EXPECT_TRUE(engine.running);
+  ASSERT_TRUE(engine.running);
 
   FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engine
                                                                                 nibName:nil
@@ -254,7 +254,7 @@ + (void)registerWithRegistrar:(id<FlutterPluginRegistrar>)registrar {
 
   // Launch the test entrypoint.
   EXPECT_TRUE([engine runWithEntrypoint:@"backgroundTest"]);
-  EXPECT_TRUE(engine.running);
+  ASSERT_TRUE(engine.running);
 
   FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engine
                                                                                 nibName:nil
@@ -476,7 +476,7 @@ + (void)registerWithRegistrar:(id<FlutterPluginRegistrar>)registrar {
 
   FlutterEngine* engine = GetFlutterEngine();
   EXPECT_TRUE([engine runWithEntrypoint:@"nativeCallback"]);
-  EXPECT_TRUE(engine.running);
+  ASSERT_TRUE(engine.running);
 
   latch.Wait();
   ASSERT_TRUE(latch_called);