selftests: mptcp: fix backport issues

By accident, some patches modifying the MPTCP selftests have been
applied twice, using different versions of the patch [1].

These patches have been dropped, but it looks like quilt incorrectly
handled that by placing the new subtests at the wrong place: in
userspace_tests() instead of endpoint_tests(). That caused a few other
patches not to apply properly.

Not to have to revert and re-apply patches, this issue can be fixed by
moving some code around.

Link: https://lore.kernel.org/fc21db4a-508d-41db-aa45-e3bc06d18ce7@kernel.org

 [1]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>