selftests: tls: add getsockopt test

The kernel accepts fetching either just the version and cipher type,
or exactly the per-cipher struct. Also check that getsockopt returns
what we just passed to the kernel.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/81a007ca13de9a74f4af45635d06682cdb385a54.1692977948.git.sd@queasysnail.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>