tests: Run tagged tests with `--changed` when shared contexts change

- When a shared context file changes, the tests that include that shared
  context should be run when `brew tests --changed` is invoked. This
  ensures that relevant test failures are caught.
- I found this bug while working on PR 22469 where I changed shared
  contexts, ran `brew lgtm` which ran `brew tests --changed` which
  lulled me into a false sense of security that the changes were fine,
  until CI ran the full test suite[1].
- Add some standard behaviour tests for `brew tests --changed` as well as
  there was zero coverage before.

[1]: https://github.com/Homebrew/brew/pull/22469#issuecomment-4583056266