ci: create a test:slow script to avoid re-running fast tests in the slow-tests job

[erikian]

… slow-tests job

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:
Basically the title. This PR slashes a minute or so (~10%-ish) from the time it takes to run the slow tests by not re-running the fast tests, which are already covered by a separate job.

[erickzhao]

seems legit thanks!