selftests: forwarding: Change inappropriate log_test_skip() calls

The SKIP return should be used for cases where tooling of the machine under
test is lacking. For cases where HW is lacking, the appropriate outcome is
XFAIL.

This is the case with ethtool_rmon and mlxsw_lib. For these, introduce a
new helper, log_test_xfail().

Do the same for router_mpath_nh_lib. Note that it will be fixed using a
more reusable way in a following patch.

For the two resource_scale selftests, the log should simply not be written,
because there is no problem.

Cc: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/3d668d8fb6fa0d9eeb47ce6d9e54114348c7c179.1711464583.git.petrm@nvidia.com


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