Fixes thread safety issue that could cause memory corruption on ports
with (MICROPY_PY_THREAD && !MICROPY_PY_THREAD_GIL) - currently only rp2 and
unix have this configuration.
Adds unit test for TLS sockets that exercises this code path. I wasn't
able to make this fail on rp2, the race condition window is pretty narrow
and may not have a direct impact on a quiet system.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
