selftests: lib: Define more kselftest exit codes

The following patches will operate with more exit codes besides
ksft_skip. Add them here.

Additionally, move a duplicated skip exit code definition from
forwarding/tc_tunnel_key.sh. Keep a similar duplicate in
forwarding/devlink_lib.sh, because even though lib.sh will have
been sourced in all cases where devlink_lib is, the inclusion is not
visible in the file itself, and relying on it would be confusing.

Cc: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/545a03046c7aca0628a51a389a9b81949ab288ce.1711464583.git.petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/tc_tunnel_key.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
-# Kselftest framework requirement - SKIP code is 4.
-ksft_skip=4
 
 ALL_TESTS="tunnel_key_nofrag_test"
 

tools/testing/selftests/net/lib.sh

@@ -8,8 +8,12 @@
 
 BUSYWAIT_TIMEOUT=$((WAIT_TIMEOUT * 1000)) # ms
 
-# Kselftest framework requirement - SKIP code is 4.
+# Kselftest framework constants.
+ksft_pass=0
+ksft_fail=1
+ksft_xfail=2
 ksft_skip=4
+
 # namespace list created by setup_ns
 NS_LIST=""