selftests: mptcp: validate userspace PM tests by default

The new script was not listed in the programs to test.

By consequence, some CIs running MPTCP selftests were not validating
these new tests. Note that MPTCP CI was validating it as it executes all
.sh scripts from 'tools/testing/selftests/net/mptcp' directory.

Fixes: 259a834f ("selftests: mptcp: functional tests for the userspace PM type")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>