selftests: mptcp: simult flows: fix some subtest names

The selftest was correctly recording all the results, but the 'reverse
direction' part was missing in the name when needed.

It is important to have a unique (sub)test name in TAP, because some CI
environments drop tests with duplicated name.

Fixes: 675d9933 ("selftests: mptcp: simult flows: format subtests results in TAP")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>