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
