test: replace symlinks with actual unit files

test: replace symlinks with actual unit files

During installation, meson complains:
> Installing /home/zbyszek/src/systemd-work/test/units/sysinit.target to /var/tmp/systemd-test.Q1FSuj/root/usr/lib/systemd/tests/testdata/units
> Warning: trying to copy a symlink that points to a file. This will copy the file,
> but this will be changed in a future version of Meson to copy the symlink as is. Please update your
> build definitions so that it will not break when the change happens.

It *is* convenient to have those files as symlinks, but it is also confusing,
because symlinks create aliases, and it seems that in those cases we actually
don't want aliases (at least in the case of loopy*.service that'd make the test
pointless).