virt-aa-helper-test: Silence ls

virt-aa-helper-test: Silence ls

virt-aa-helper checks presence of files before it adds them into
a profile. Because of that, test cases inside of
virt-aa-helper-test that require presence of /boot/initrd* are
guarded by a check. The check uses ls to find at least one initrd
file. If there's none, then ls prints an error onto stderr. This
is not helpful because the test script prints a message on its
own right after.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>