Add script to run tests through combinations of `Sys.CPU_THREADS` and `Threads.nthreads()`

[chriselrod]

CI is slow.
Might be faster to also execute this script on CI instead of starting a bunch of short jobs, so things like installing packages and precompiling only happen once.

CI is spending a lot of time waiting for jobs to start, so because we aren't getting parallelism in this setup, we are paying that cost.

[chriselrod]

https://docs.github.com/en/actions/learn-github-actions/usage-limits-billing-and-administration#usage-limits

I wouldn't mind paying for more, but that may prompt me to kick everyone else out of the JuliaSIMD org, ha ha (you pay per user in an org).