Add `WoltModalSheet.modalBarrierColor` tests

[TahaTesser]

Related Prepare repository for tests requirement

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] My PR includes tests for all changed/updated/fixed behaviors.
• [x] All existing and new tests are passing.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] The package compiles with the minimum Flutter version stated in the pubspec.yaml

Breaking Change

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