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 <[e-mail]>
Signed-off-by: Petr Machata <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

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

···
 
 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=""