Stopped mocking the a flutter engine to make sure we delete the FlutterViewController.

[gaaclarke]

Description

Somehow the mock engine is stopping the FlutterViewController from being dealloc'd. Stop mocking clears it out. I'm not sure if it's a sign of a deeper problem.

This leaked view controller would get notifications from other tests and crash.

Related Issues

flutter/flutter#72107

Tests

I added the following tests:

It is a test.

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[dnfield]

LGTM