test: move test units to their own directories

This also add test cases for verifying loopy.service and friends,
which was added by e66047ff62 but never
used in integration tests.

test/test-engine/basic.target

@@ -0,0 +1,22 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Basic System
+Documentation=man:systemd.special(7)
+Requires=sysinit.target
+Wants=sockets.target timers.target paths.target slices.target
+After=sysinit.target sockets.target paths.target slices.target tmp.mount
+
+# We support /var, /tmp, /var/tmp, being on NFS, but we don't pull in
+# remote-fs.target by default, hence pull them in explicitly here. Note that we
+# require /var and /var/tmp, but only add a Wants= type dependency on /tmp, as
+# we support that unit being masked, and this should not be considered an error.
+RequiresMountsFor=/var /var/tmp
+Wants=tmp.mount