ToggleButtons test improvement

[shihaohong]

Description

This improves the ToggleButtons test files to have more precise finders looking for DefaultTextStyle and IconTheme. Instead of using the first ancestor widget looking up the tree (there will be 2+ widgets), it looks for the single descendant widget of the RawMaterialButton.

Related Issues

n/a

Tests

The modified lines improve existing tests

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[HansMuller]

Maybe just define a local function that returns a text style. Like:

TextStyle buttonTextStyle(String text) {
  return tester.widget<DefaultTextStyle>(
    find.descendant(
      of: find.widgetWithText(RawMaterialButton, text),
      matching: find.byType(DefaultTextStyle),
    )
  ).style;
}

[HansMuller]

LGTM