Fix tests that rely on FadeTransition incorrectly and depend on implementation details of FadeUpwardsTransitionBuilder

[shihaohong]

Description

There are a few tests that depend on the page transition theme's default value for TargetPlatform.android, which blocks updating the default activity transitions. This PR sets the page transition for particular tests that rely on the FadeUpwardsTransitionBuilder's implementation details, while fixing tests that were incorrectly looking for a FadeTransition widget or searching the
entire widget tree when it is unnecessary.

This isn't a breaking change because no actual implementation code has been changed, even though the tests were modified.

Related Issues

Fixes #51145

Tests

1. This PR fixes broken tests that incorrectly assume the presence of FadeTransition/Opacity widgets (BottomNavigationBar tests)
2. This PR removes reliance on default PageTransitionsTheme values where appropriate (every other test)

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Breaking Change

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[justinmc]

LGTM, solid improvement!