selftests: set amt.sh executable

amt.sh test script will not work because it doesn't have execution
permission. So, it adds execution permission.
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Fixes: c08e8bae ("selftests: add amt interface selftest script")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20220105144436.13415-1-ap420073@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>