test: add a custom SELinux file context

Since the test suite overhaul, the test units are now under
/usr/lib/systemd/tests/testdata/tetsuite-06.units with
system_u:object_r:lib_t context. This causes an AVC denial, since the
systemd unit files are expected to have the
system_u:object_r:systemd_unit_file_t context. Let's fix this by using a
custom file context definition.