test: reader_concurrency_test: add reader_concurrency_semaphore_dump_reader_diganostics
test: reader_concurrency_test: add reader_concurrency_semaphore_dump_reader_diganostics

Not really testing anything, at least not automatically. It just
provides coverage for the diagnostics dump code, as well as allows for
developers to inspect the printout visually when making changes.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla