feat: Added support for tests

[kaomoneus]

Description

Added support for tests. Namely introduced a way of how to check whether overlay widget is present or not.

This is done by specifying overlay widget key, which is a ValueKey of global key provided by user for his Showcase widget (see test from this PR for more details).

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs: etc).
• I have followed the [Contributor Guide] when preparing my PR.
• 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 or docs.

Migration instructions

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[aditya-css]

@kaomoneus Thanks for your contribution. Can you please rebase your branch and remove the android changes. Thanks!

[kaomoneus]

@aditya-css , done!

[aditya-css]

Hey @kaomoneus,
I regret to inform you that I would need to close this PR since we have done radical improvements to this package that has changed the code structure as well. I'll ensure that testability is included. We would love for you to do the contribution for the same.