Fix tests using palettize

[pnuu]

After merging pytroll/trollimage#179 and pytroll/trollimage#181 and creating a new Trollimage release, two tests using palettize and water_detection (also does palettize) were failing because of the added _FillValue attribute. The fix gets a bit ugly because colorize also passes palettes but doesn't add the _FillValue attribute.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.09%. Comparing base (d62b5d2) to head (b9a2f4d).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2944      +/-   ##
==========================================
- Coverage   96.11%   96.09%   -0.03%     
==========================================
  Files         377      377              
  Lines       55067    55070       +3     
==========================================
- Hits        52929    52920       -9     
- Misses       2138     2150      +12     

Flag	Coverage Δ
behaviourtests	3.94% <0.00%> (-0.01%)	⬇️
unittests	96.19% <100.00%> (-0.03%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 11437490447

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.202%

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Totals
Change from base Build 11403042925:	0.0%
Covered Lines:	53164
Relevant Lines:	55263

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💛 - Coveralls

[gerritholl]

Looks good to me.