selftests: mptcp: lib: support flaky subtests

Some subtests can be unstable, failing once every X runs. Fixing them
can take time: there could be an issue in the kernel or in the subtest,
and it is then important to do a proper analysis, not to hide real bugs.

To avoid creating noises on the different CIs, it is important to have a
simple way to mark subtests as flaky, and ignore the errors. This is
what this patch introduces: subtests can be marked as flaky by setting
MPTCP_LIB_SUBTEST_FLAKY env var to 1, e.g.

  MPTCP_LIB_SUBTEST_FLAKY=1 <run flaky subtest>

The subtest will be executed, and errors (if any) will be ignored. It is
still good to run these subtests, as it exercises code, and the results
can still be useful for the on-going investigations.

Note that the MPTCP CI will continue to track these flaky subtests by
setting SELFTESTS_MPTCP_LIB_OVERRIDE_FLAKY env var to 1, and a ticket
has to be created before marking subtests as flaky.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240524-upstream-net-20240524-selftests-mptcp-flaky-v1-1-a352362f3f8e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>