selftests: mptcp: pm nl: skip fullmesh flag checks if not supported

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

One of them is the fullmesh flag that can be given to the MPTCP
in-kernel path-manager and introduced in commit 2843ff6f ("mptcp:
remote addresses fullmesh").

If the flag is not visible in the dump after having set it, we don't
check the content. Note that if we expect to have this feature and
SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES env var is set to 1, we always
check the content to avoid regressions.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368


Fixes: 6da1dfdd ("selftests: mptcp: add set_flags tests in pm_netlink.sh")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>