Port ClassDB tests to use doctest

Extracted the most minimal core initialization functionality from
`setup()` and `setup2()` so that `ClassDB` could be tested properly
(input, audio, rendering, physics etc, are excluded).

Display and rendering servers/singletons are not initialized at all.

Due to the fact that most subsystems are disabled, fixed various crashes in the
process (in order):
- `AcceptDialog` OK/cancel swap behavior (used `DisplayServer` while
  `register_scene_types()`);
- `make_default_theme` which depends on `RenderingServer`;
- `XRServer` singleton access while calling `register_modules_types()`;
- hidden bug in a way joypads are cleaned up (MacOS and Linux only).

Removed manual `ClassDB` init/cleanup calls from `test_validate_testing.h`.

ClassDB tests:

Co-authored-by: Ignacio Etcheverry <ignalfonsore@gmail.com>

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