selftests/bpf: less spam in the log for message matching

selftests/bpf: less spam in the log for message matching

When running test_loader based tests in the verbose mode each matched
message leaves a trace in the stderr, e.g.:

    ./test_progs -vvv -t ...
    validate_msgs:PASS:expect_msg 0 nsec
    validate_msgs:PASS:expect_msg 0 nsec
    validate_msgs:PASS:expect_msg 0 nsec
    validate_msgs:PASS:expect_msg 0 nsec
    validate_msgs:PASS:expect_msg 0 nsec

This is not very helpful when debugging such tests and clobbers the
log a lot.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240820102357.3372779-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>