Improve tests

[mducle]

Reduces the verbosity of the tests by setting fid to 0 to suppress output and removing a lot of warnings.

Also:

• Fix bug where system test data was always being regenerated (and hence always passing). This only caused one test (to do with twins) to fail because we changed the twin rotc matrix definition compared to the reference data. Currently this is "fixed" by forcing the test to use the old twin matrix.
• Fix several "flaky" test by zero-initialising arrays in chol_omp mex file.
• Use fixed seed for RNG in optmagk to avoid occasional test failures as it converges to a local minimum.
• Uses persistent for swpref in sw_timeit to avoid performance hit

[codecov-commenter]

Codecov Report

Merging #138 (c1aac00) into master (8f4eecf) will decrease coverage by 0.31%.
The diff coverage is 41.17%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
- Coverage   38.98%   38.67%   -0.31%     
==========================================
  Files         239      239              
  Lines       15829    15829              
==========================================
- Hits         6171     6122      -49     
- Misses       9658     9707      +49     

Impacted Files	Coverage Δ
swfiles/@swpref/private/datastruct.m	80.19% <ø> (+1.15%)	⬆️
swfiles/sw_timeit.m	8.97% <33.33%> (-50.49%)	⬇️
swfiles/@spinw/genmagstr.m	100.00% <100.00%> (ø)
swfiles/sw_cartesian.m	65.51% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[github-actions]

Test Results

       4 files  ±0       98 suites  ±0   18m 18s ⏱️ + 1m 32s
   583 tests ±0     583 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
1 622 runs  ±0  1 622 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit c1aac00. ± Comparison against base commit 8f4eecf.

♻️ This comment has been updated with latest results.

[RichardWaiteSTFC]

Looks great - thanks!