tests: benchmarks: move pthread_pressure to benchmarks/posix

The pthread_pressure test was not a typical test per se. It was
a benchmark in search of the proper home.

Let's move it to the correct place in the Zephyr tree, add a
doc, and provide some reporting.

Currently, k_threads out-perform pthreads by almost a factor of
2. The theoretical maximum performance of pthreads would be at
parity of k_threads, since pthreads are a wrapper around kernel
threads. It would be great to reduce the gap.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>