selftests: mptcp: more operations in ns_init/exit

Set more the default sysctl values in mptcp_lib_ns_init(). It is fine to
do that everywhere, because they could be overridden latter if needed.

mptcp_lib_ns_exit() now also try to remove temp netns files used for the
stats even for selftests not using them. That's fine to do that because
these files have a unique name.

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/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-5-bc79e6e5e6a0@kernel.org


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