oeqa/selftest: extend overlayfs test

Test that overlayfs.bbclass generates one systemd unit, that
applications can set dependencies on, and that this unit mounts all
required overlays

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>