test-execute: make find invocation a bit more efficent, increase timeout

We go through the whole file system, so this test can take arbitrary time.  But
this test is still quite useful, so let's at least try to make it more efficent
by not descending at all into the directories we would filter out later on
anyway.

Also increase the timeout, in case the previous step doesn't help enough.

src/test/meson.build

@@ -566,7 +566,8 @@ tests += [
           libseccomp,
           libselinux,
           libmount,
-          libblkid]],
+          libblkid],
+         '', 'timeout=360'],
 
         [['src/test/test-siphash24.c'],
          [],