selftests: mptcp: more stable join tests-cases

MPTCP join self-tests are a bit fragile as they reply on
delays instead of events to catch-up with the expected
sockets states.

Replace the delay with state checking where possible and
reduce the number of sleeps in the most complex scenarios.

This will both reduce the tests run-time and will improve
stability.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>