run-testables ends up being called within its own body, which
kaocha is parallelizing. We generally want to parallelize at the
namespace level, so we use run-test-serial for other testable types.
This way we only create one threadpool at a time.

The previous code references fixtures/f-tests, but there is no such
directory. The test case looks unfinished, so I'm not sure that there
ever was a working f-tests dir.

To fix the test case, I have adapted the code from run-test, just before
run-test-parallel.