selftests: mptcp: lib: add time per subtests in TAP output

selftests: mptcp: lib: add time per subtests in TAP output

It adds 'time=<N>ms' in the diagnostic data of the TAP output, e.g.

  ok 1 - pm_netlink: defaults addr list # time=9ms

This addition is useful to quickly identify which subtests are taking a
longer time than the others, or more than expected.

Note that there are no specific formats to follow to show this time
according to the TAP 13 [1], TAP 14 [2] and KTAP [3] specifications.
Let's then define this one here.

Link: https://testanything.org/tap-version-13-specification.html
Link: https://testanything.org/tap-version-14-specification.html
Link: https://docs.kernel.org/dev-tools/ktap.html
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240906-net-next-mptcp-ksft-subtest-time-v2-1-31d5ee4f3bdf@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>