CI: Quadlet rootfs test: use container image as rootfs

Test was written to use / (root). This is not parallel-safe.

Fixes: #23909

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/system/252-quadlet.bats

@@ -723,10 +723,18 @@ EOF
 @test "quadlet - rootfs" {
     skip_if_no_selinux
     skip_if_rootless
+
+    # Mount a container image to use as rootfs. Because we (may) run in
+    # parallel, mount a working container, not $IMAGE
+    cname="c-$(safename)"
+    run_podman run -d --name $cname $IMAGE top
+    run_podman container mount $cname
+    mountpoint="$output"
+
     local quadlet_file=$PODMAN_TMPDIR/basic_$(safename).container
     cat > $quadlet_file <<EOF
 [Container]
-Rootfs=/:O
+Rootfs=$mountpoint:O
 Exec=sh -c "echo STARTED CONTAINER; echo "READY=1" | socat -u STDIN unix-sendto:\$NOTIFY_SOCKET; top -b"
 Notify=yes
 EOF
@@ -735,6 +743,11 @@ EOF
     service_setup $QUADLET_SERVICE_NAME
 
     wait_for_output "STARTED CONTAINER" $QUADLET_CONTAINER_NAME
+
+    # Done. Clean up.
+    service_cleanup $QUADLET_SERVICE_NAME failed
+    run_podman container unmount $cname
+    run_podman rm -f -t0 $cname
 }
 
 @test "quadlet - selinux disable" {