[libc] Default to a single threaded thread pool for GPU tests (#74486)

Summary:
The GPU tests tend to fail when run massively in parallel. This is why
we use a CMake job pool to limit it to 1 in most cases. We should
default to the configuration that is most likely to work, that being a
single thread. There aren't enough GPU tests for this to be a massive
increase in test time on the bots, so we should default to what works
guaranteed.