properties/tests: use different TMPDIR for testing libnm/libnm-glib based tests

We compile two versions of the properties tests from the same source file:
one for libnm and one for legacy libnm-glib. When running them in parallel
with `make -j 4 check`, we must ensure that the tests use different temporary
directories.

https://bugzilla.gnome.org/show_bug.cgi?id=772471