refactor: Improve tests

[st-pasha]

Description

• Modernize various tests (helper classes made private, proper name of test groups, use testWithFlameGame where appropriate, test for assert failure messages);
• For some tests improved coverage;
• Fixed bug with ParentIsA mixin, where wrong parent triggered a TypeCast error instead of an assertion error. The corresponding test couldn't catch this because it tried to call onMount manually instead of actually mounting the component, and the error was thrown before the mounting stage.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• [-] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples.

Breaking Change

• [-] Yes, this is a breaking change.
• No, this is not a breaking change.

Related Issues

WIP for #1351

[spydon]

Are there no withX.test left now?

[st-pasha]

Are there no withX.test left now?

There are probably quite a few remains -- it takes time to go over every test.