test/system: fix mount external container test

Checking for the mountdir is not relevent, a recent c/storage change[1] no
longer deletes the mount point directory so the check will cause a false
positive. findmnt exits 1 when the given path is not a mountpoint so
let's use that to check.

[1] 3f2e81abb3

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

test/system/060-mount.bats

@@ -249,13 +249,10 @@ EOF
     reported_mountpoint=$(echo "$output" | awk '{print $2}')
     is "$reported_mountpoint" "$mount_path" "mountpoint reported by 'podman mount'"
 
-    # umount, and make sure files are gone
+    # umount, and make sure mountpoint no longer exists
     run_podman umount $external_cid
-    if [ -d "$mount_path" ]; then
-        # Under VFS, mountpoint always exists even despite umount
-        if [[ "$(podman_storage_driver)" != "vfs" ]]; then
-            die "'podman umount' did not umount $mount_path"
-        fi
+    if findmnt "$mount_path" >/dev/null ; then
+        die "'podman umount' did not umount $mount_path"
     fi
     buildah rm $external_cid
 }