selftests: pmtu.sh: improve the test result processing

This test will treat all non-zero return codes as failures, it will
make the pmtu.sh test script being marked as FAILED when some
sub-test got skipped.

Improve the result processing by
  * Only mark the whole test script as SKIP when all of the
    sub-tests were skipped
  * If the sub-tests were either passed or skipped, the overall
    result will be PASS
  * If any of them has failed with return code 1 or anything bad
    happened (e.g. return code 127 for command not found), the
    overall result will be FAIL

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>