I spent too much time trying to navigate validate Python code to figure
out where a test was coming from. Hoping that it's slightly easier for
the next person, this patch:
* Adds type annotations to setup_tests(), for the sake of code
navigation.
* Adds comments matching each test generator with the patterns of test
names it produces.
* Removes an if statement in `register_default_scenarios()` where both
branches have the same exact code with the same exact very long list.
* Removes NamedDic [sic] and replaces it with SimpleNamespace from the
standard library (3.3+) which has the same purpose and API.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8090>
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