test: use /proc/mounts instead of df+awk to check /tmp mount

awk is not available in all CI containers. /proc/mounts is a standard
Linux procfs file with no external tool dependencies; grep on its second
field reliably detects whether /tmp is its own top-level mount point.
On macOS/BSD where /proc/mounts does not exist the grep exits non-zero
and the test skips silently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: andy5995

test/test_media_root.sh

@@ -21,12 +21,10 @@ else
 fi
 
 # This test requires /tmp to be a top-level mount point on its own device so
-# that rmw will write a relative Path in the trashinfo. Skip if that condition
-# isn't met (e.g. containers where /tmp is on the root filesystem, or macOS
-# where /tmp is a symlink into the root partition).
-tmp_mount=$(df -P /tmp 2>/dev/null | awk 'NR==2 {print $NF}')
-if [ "$tmp_mount" != "/tmp" ]; then
-  echo "/tmp is not a top-level mount point (got: ${tmp_mount:-unavailable}); skipping"
+# that rmw will write a relative Path in the trashinfo. Check /proc/mounts
+# (Linux); on macOS/BSD where it doesn't exist the grep fails and we skip.
+if ! grep -q '^[^ ]* /tmp ' /proc/mounts 2>/dev/null; then
+  echo "/tmp is not a top-level mount point; skipping"
   exit 0
 fi