MRG: extend tests, with some fixes

[matthew-brett]

So far, behavior changes are:

• Change in algorithm for writing qform and sform affines. We now write as many affines as we have, preferring sform for affines further through processing.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.07407% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.57%. Comparing base (f47b49b) to head (58841e9).

Files	Patch %	Lines
src/xibabel/tests/test_loaders.py	99.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #20 +/- ## ========================================== + Coverage 90.86% 92.57% +1.70% ========================================== Files 12 12 Lines 1062 1131 +69 Branches 160 163 +3 ========================================== + Hits 965 1047 +82 + Misses 72 64 -8 + Partials 25 20 -5 ```

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