[macOS] Clean up resources in ViewController tests

[cbracken]

Wraps all FlutterViewController tests in an autorelease pool to ensure resources are cleaned up.

Adds a MockFlutterEngineTest subclass of AutoreleasePoolTest that creates an OCPartialMock FlutterEngine and shuts it down at the end of the test. Previously we were not shutting down any FlutterEngine instances we allocated, resulting in potentially thousands of threads and graphics contexts being allocated.

Prior to this change, running these tests via:

../out/host_debug_unopt_arm64/flutter_desktop_darwin_unittests \
    --gtest_filter='FlutterViewController.*' --gtest_repeat=1000

resulted in test failures and sometimes segfaults. This ensures resources are cleaned up

Eventually all unit tests should configure their FlutterEngine via either FlutterEngineTest (which should be an AutoreleasePoolTest) or MockFlutterEngineTest, and the CreateMockFlutterEngine function moved to a static used in the implementation of these.

Issue: flutter/flutter#104789
Issue: flutter/flutter#127441
Issue: flutter/flutter#124840

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[dkwingsmt]

LGTM, but why do we sometimes just pass an engineMock to the test, sometimes we put it in a finally block?

[cbracken]

Nice catch! That was the first test where I experimented with this. I completely forgot to clean it up. Fixed.

The very last test is a special case since we intentionally allocate things within a manually-specified autoreleasepool (you might need to expand the diff).