The thread pool test is currently flakey and takes over 2 minutes to run
on CI. It also currently has no users now that RequestServer uses curl,
so let's just remove it for now. If we need it in the future, we can
revive it from git history.
Using a file(GLOB) to find all the test files in a directory is an easy
hack to get things started, but has some drawbacks. Namely, if you add
a test, it won't be found again without re-running CMake. `ninja` seems
to do this automatically, but it would be nice to one day stop seeing it
rechecking our globbed directories.