tests: fix access mode of root inode of throw-away container images

Otherwise the root inode will typically have what mkdtemp sets up, which
is something like 0700, which is weird and somewhat broken when trying
to look into containers from unpriv users.

test/units/util.sh

@@ -182,6 +182,7 @@ create_dummy_container() {
     fi
 
     mkdir -p "$root"
+    chmod 555 "$root"
     cp -a /usr/share/TEST-13-NSPAWN-container-template/* "$root"
     coverage_create_nspawn_dropin "$root"
 }