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 <[email protected]>
Fixes: c08e8ba ("selftests: add amt interface selftest script")
Signed-off-by: Taehee Yoo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: TaeheeYoo