Add and run tests/network-config-tests-diff

This test runs ./tests/network-config-tests (which attempts to configure every built-in chutney network), first at HEAD, then at a specified commit (using a cloned repo in a tmpdir). Both runs are executed under shadow to get determinism. Finally the outputs are compared using diffoscope.

In CI the test fails if there is any diff, but the job is marked as allow_failure: true. The intent is to raise awareness when there is a diff, and show what it is, but not to block merging.

The test can also be run locally, assuming diffoscope and shadow are installed.

Fixes #40027 (closed)