libnm/tests: move common testing code to nm-test-libnm-utils

libnm/tests: move common testing code to nm-test-libnm-utils

The unit tests for libnm and libnm-glib use a NetworkManager stub
service written in Python (test-networkmanager-service.py). As they
share the same server, it makes sense to also share the same utility
code to drive the stub.

Move the common code to include/.

Note that contrary to "nm-test-utils.h", "nm-test-libnm-utils.h" is not
a header-only file. Instead its implementation is in "nm-test-utils-impl.c".
The reason for that this split is, if we later have yet another non-header-only
test-utility, then all the implementations are in "nm-test-utils-impl.c", requiring
the tests to link only one object file.