[test] Adding missing partition tests

[LucianoPAlmeida]

I just note that partition(subrange:) both BidirectionalCollection(specialized) and MutableCollection overloads. So this just adds the tests :)

cc @natecook1000 @timvermeulen @kylemacomber

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [ ] I've read the Contribution Guidelines
• [ ] I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test