selftests: net: lib: Do not overwrite error messages

ret_set_ksft_status() calls ksft_status_merge() with the current return
status and the last one. It treats a non-zero return code from
ksft_status_merge() as an indication that the return status was
overwritten by the last one and therefore overwrites the return message
with the last one.

Currently, ksft_status_merge() returns a non-zero return code even if
the current return status and the last one are equal. This results in
return messages being overwritten which is counter-productive since we
are more interested in the first failure message and not the last one.

Fix by changing ksft_status_merge() to only return a non-zero return
code if the current return status was actually changed.

Add a test case which checks that the first error message is not
overwritten.

Before:

# ./lib_sh_test.sh
[...]
TEST: RET tfail2 tfail -> fail [FAIL]
retmsg=tfail expected tfail2
[...]
# echo $?
1

After:

# ./lib_sh_test.sh
[...]
TEST: RET tfail2 tfail -> fail [ OK ]
[...]
# echo $?
0

Fixes: 596c8819cb78 ("selftests: forwarding: Have RET track kselftest framework constants")
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20251116081029.69112-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>