tests: Update Python stanza to change directories before running tests

Ensure PYTHONPATH uses absolute paths for top_builddir and top_srcdir. Use
pushd/popd before and after running tests with "python -m unittest...". This
ensures TESTARGS can be used from within a vpath build directory.

https://bugzilla.gnome.org/show_bug.cgi?id=720713
tests: Use Automake test harness for tests/offsets

Replace usage of check-local with the Automake test harness. Change check_
variable usage to EXTRA_ in order to avoid building these dependencies with
targeted testing using TESTS= in parent directories.

https://bugzilla.gnome.org/show_bug.cgi?id=720713