selftests: mptcp: print test results with colors

To unify the output formats of all test scripts, this patch adds
four more helpers:

	mptcp_lib_pr_ok()
	mptcp_lib_pr_skip()
	mptcp_lib_pr_fail()
	mptcp_lib_pr_info()

to print out [ OK ], [SKIP], [FAIL] and 'INFO: ' with colors. Use them
in all scripts to print the "ok/skip/fail/info' using the same 'format'.

Having colors helps to quickly identify issues when looking at a long
list of output logs and results.

Note that now all print the same keywords, which was not the case
before, but it is good to uniform that.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-9-4f42c347b653@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>