tests: Don't try to create more than one Renderer for the same Window

[smcv]

The SDL API is that it's a programming error to mix the Renderer API with use of a window surface. Some Renderer implementations create a window surface, making it invalid to attach more than one Renderer to the same window. SDL 2.28 is better at detecting this than previous versions were, causing some of py-sdl2's unit tests to fail with:

Error encountered creating the SDL renderer: Surface already associated with window

when running with the dummy video backend and software renderer (which are used in Debian as automated tests to check for obvious regressions).

Some instances of this were already fixed in 0.9.15, most likely as part of "Try fixing renderer tests w/ dummy on Windows". For the remaining failing cases, avoid the failure by creating a new Window for each part of the test.

Bug-Debian: https://bugs.debian.org/1038823

Merge Checklist

• [ ] the PR has been reviewed and all comments are resolved
• [ ] all CI checks pass
• [ ] (if applicable): the PR description includes the phrase closes #<issue-number> to automatically close an issue
• [ ] (if applicable): bug fixes, new features, or API changes are documented in news.rst

[a-hurst]

Nice, thanks for this! One less thing to tackle when updating the bindings to 2.28.