tests: Add tests for the gdbus-codegen executable

Add a Python-based test wrapper for the gdbus-codegen executable, similar to the existing tests for glib-mkenums and friends.

Add a few basic tests to begin with, but this doesn’t approach anywhere near full coverage.

The next step is to move the existing Meson-based gdbus-codegen tests from gio/tests/meson.build into the Python test suite.

Signed-off-by: Philip Withnall withnall@endlessm.com

Helps: #1612