selftests: mptcp: allow changing subtests prefix

If a CI executes the same selftest multiple times with different
options, all results from the same subtests will have the same title,
which confuse the CI. With the same title printed in TAP, the tests are
considered as the same ones.

Now, it is possible to override this prefix by using MPTCP_LIB_KSFT_TEST
env var, and have a different title.

While at it, use 'basename' to remove the suffix as well instead of
using an extra 'sed'.

Fixes: c4192967 ("selftests: mptcp: lib: format subtests results in TAP")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240131-upstream-net-20240131-mptcp-ci-issues-v1-7-4c1c11e571ff@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>