chore: Enable DCM rule avoid-top-level-members-in-tests

[luanpotter]

Description

Enable DCM rule avoid-top-level-members-in-tests.

Since tests shouldn't import other tests, this rule makes sense. In fact there were a couple of mistakes I caught while fixing.

Checklist

• [x] I have followed the Contributor Guide when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

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