bpo-43592: Raise RLIMIT_NOFILE in test.libregrtest (GH-29127)

Raise RLIMIT_NOFILE in test.libregrtest.

On macOS the default is often too low for our testsuite to succeed.

Co-authored by reviewer: Victor Stinner
(cherry picked from commit 843b890334ca30cf6af27dffe29cecd06b49f7d9)

Co-authored-by: Gregory P. Smith <greg@krypto.org>