Adjust and refactor all OutlineButton tests into its respective file

[Inconnu08]

Description

The existing monolithic buttons_tests.dart file contains tests for different buttons like FlatButton, RaisedButton, OutlineButton and MaterialButton, even though flat_button_test.dart, raised_button_test.dart, outline_button_test.dart, and material_button_test.dart exists.

This PR splits up and adjusts OutlineButton tests properly into its respective file.

Related Issues

#42798

Tests

I added the following tests:

• Refactors existing

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[shihaohong]

LGTM! Thanks for doing the refactor :)

[Inconnu08]

My pleasure :)