Started tearing down the mock engine in tearDown in FlutterViewControllerTest

[gaaclarke]

Description

Turns out that the view controllers were leaking in other tests so I moved the stopMocking into the tearDown method.

Related Issues

flutter/flutter#72107
#23013

Tests

I added the following tests:

Fixes tests.

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[dnfield]

Can you add a comment about why we're doing this - especially since we're doing it for this mock and not others?

[gaaclarke]

Done.

[dnfield]

LGTM with nit about comment

[gaaclarke]

Merging before new round of tests pass, just adding a comment since last pass of all the tests.