Add test coverage for pretuning and fix support for pretuning with no…

[anarkiwi]

…n-offload FFT.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.11% 🎉

Comparison is base (5ba7f16) 75.69% compared to head (9f082ac) 75.81%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #878      +/-   ##
==========================================
+ Coverage   75.69%   75.81%   +0.11%     
==========================================
  Files          41       41              
  Lines        3469     3469              
==========================================
+ Hits         2626     2630       +4     
+ Misses        843      839       -4     

Files Changed	Coverage Δ
gamutrf/grscan.py	80.34% <100.00%> (+3.03%)	⬆️
tests/test_grscan.py	100.00% <100.00%> (ø)

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