Drop 'S' from end of CONFIG_MPTCP_KUNIT_TESTS in order to adhere to the
KUNIT *_KUNIT_TEST config name format.

Fixes: a00a5822 (mptcp: move crypto test to KUNIT)
Reviewed-by: NDavid Gow <davidgow@google.com>
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NNico Pache <npache@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>