[macOS] Wrap FlutterEngineTest in autoreleasepool (flutter#50832)

Previously, these were not freeing allocations. This resulted in a lot
of running engines/threads which triggered issues with running out of
resources (Metal resources in particular).

This also re-enables two disabled unit tests for view background colour.

Issue: flutter#124677

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Author: cbracken

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

@@ -222,8 +222,7 @@ @implementation MockableFlutterEngine
   EXPECT_TRUE(stdout_capture.GetOutput().find("Hello logging") != std::string::npos);
 }
 
-// TODO(cbracken): Needs deflaking. https://github.com/flutter/flutter/issues/124677
-TEST_F(FlutterEngineTest, DISABLED_BackgroundIsBlack) {
+TEST_F(FlutterEngineTest, BackgroundIsBlack) {
   FlutterEngine* engine = GetFlutterEngine();
 
   // Latch to ensure the entire layer tree has been generated and presented.
@@ -252,8 +251,7 @@ @implementation MockableFlutterEngine
   latch.Wait();
 }
 
-// TODO(cbracken): Needs deflaking. https://github.com/flutter/flutter/issues/124677
-TEST_F(FlutterEngineTest, DISABLED_CanOverrideBackgroundColor) {
+TEST_F(FlutterEngineTest, CanOverrideBackgroundColor) {
   FlutterEngine* engine = GetFlutterEngine();
 
   // Latch to ensure the entire layer tree has been generated and presented.

shell/platform/darwin/macos/framework/Source/FlutterEngineTestUtils.h

@@ -15,7 +15,7 @@
 
 namespace flutter::testing {
 
-class FlutterEngineTest : public ::testing::Test {
+class FlutterEngineTest : public AutoreleasePoolTest {
  public:
   FlutterEngineTest();