selftests/bpf: Reduce test_xdp_adjust_frags_tail_grow logs

For selftest xdp_adjust_tail/xdp_adjust_frags_tail_grow, if tested failure,
I see a long list of log output like

    ...
    test_xdp_adjust_frags_tail_grow:PASS:9Kb+10b-untouched 0 nsec
    test_xdp_adjust_frags_tail_grow:PASS:9Kb+10b-untouched 0 nsec
    test_xdp_adjust_frags_tail_grow:PASS:9Kb+10b-untouched 0 nsec
    test_xdp_adjust_frags_tail_grow:PASS:9Kb+10b-untouched 0 nsec
    ...

There are total 7374 lines of the above which is too much. Let us
only issue such logs when it is an assert failure.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250607013610.1551399-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>