selftests: net: lib: Move checks from forwarding/lib.sh here

For logging to be useful, something has to set RET and retmsg by calling
ret_set_ksft_status(). There is a suite of functions to that end in
forwarding/lib: check_err, check_fail et.al. Move them to net/lib.sh so
that every net test can use them.

Existing lib.sh users might be using these same names for their functions.
However lib.sh is always sourced near the top of the file (checked), and
whatever new definitions will simply override the ones provided by lib.sh.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://patch.msgid.link/f488a00dc85b8e0c1f3c71476b32b21b5189a847.1731589511.git.petrm@nvidia.com


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