[rfw] Update tests to no longer depend on a Container pumped via testWidgets.

[ksokolovskyi]

This PR updates rfw package tests to no longer depend on a Container pumped via testWidgets to unblock further work on flutter/flutter#163419

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[ksokolovskyi]

Hi @Hixie, could you please take a look at this PR when you have time? Thanks!

[stuartmorgan-g]

Changes in the test/ directory don't need changelog and version changes, so these should be reverted.

[ksokolovskyi]

Thanks for the review, I've reverted this change in the latest commit.

[stuartmorgan-g]

LGTM

[Hixie]

A change that broke this would break tons of other tests across the community. I think we should take this as a sign that the underlying change is too dangerous to make, not as a sign that RFW's tests need changing.