TEST-08: make sure / is remounted rw

TEST-08: make sure / is remounted rw

In this test /etc/fstab is replaced by -.mount unit. This causes
systemd-remount-fs.service to not remount / rw, which in turn causes various
failures becuase /var is not writable. In particular
systemd-tmpfiles-setup.service reports many failures. This is something
to possibly fix on its own (see https://github.com/systemd/systemd/issues/791);
in the meanwhile let's fix this test so that it doesn't fail, since the
point of the test is to check aliases on mount units, and not a ro root.