CI: Diagnostic for failure in test_rebind cleanup

I saw this test fail intermittently due to what seemed like a filesystem
race in docker? The cleanup task was failing with a 'directory not
empty' error, despite trying to do a recursive 'rm'. This patch adds an
'ls' to the same directory, hoping the output might be useful to
diagnose future intermittent failures.

src/test/test_rebind.sh

@@ -51,6 +51,7 @@ tmpdir=
 # shellcheck disable=SC2317
 clean () {
   if [ -n "$tmpdir" ] && [ -d "$tmpdir" ]; then
+    ls -l "$tmpdir"
     rm -rf "$tmpdir"
   fi
 }