Re-enable `flutter test` expression evaluation tests (#36431)

Plus:

- Resume the app being run after expression has been evaluated to prevent the test run being forcefully terminated because it didn't end
- Skip non-JSON when waiting for JSON (eg. when running in verbose mode for debugging locally)
- Pass defaultTimeout in waitForJson (otherwise the assert in _timeoutWithMessages fails when it's not provided)

Closes https://github.com/flutter/flutter/issues/26518 (the original reason for skip was fixed in https://github.com/dart-lang/sdk/issues/34224).