selftests: mptcp: lib: catch duplicated subtest entries

selftests: mptcp: lib: catch duplicated subtest entries

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

When adding a new subtest entry, an error message is printed in case of
duplicated entries. If there were duplicated entries and if all features
were expected to work, the script exits with an error at the end, after
having printed all subtests in the TAP format. Thanks to that, the MPTCP
CI will catch such issues early.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-1-b6c8a10396bd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>