meson: Use rsync to copy test data directories

meson: Use rsync to copy test data directories

install_subdir() does not copy symlinks but copies the file they
point to. We also get a very ugly warning in the meson install
output:

"""
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.
"""

Let's fix both problems at once by using rsync which does the right
thing. Verified by running systemd-dissect --mtree on both the install
output before and after and all the symlinks are now correctly preserved.