Refactor creator render test functionality

[adrg]

• Extract render functionality from the creator package and add it to the testutils package so it can be used by other packages as well.
• Refactor render test helpers in the creator package.

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This change is 

[codecov[bot]]

Codecov Report

Merging #386 into development will increase coverage by 4.55%. The diff coverage is 51.11%.

@@               Coverage Diff               @@
##           development     #386      +/-   ##
===============================================
+ Coverage        52.33%   56.89%   +4.55%     
===============================================
  Files              248      249       +1     
  Lines            47161    47251      +90     
===============================================
+ Hits             24683    26883    +2200     
+ Misses           19307    16955    -2352     
- Partials          3171     3413     +242     

Impacted Files	Coverage Δ
internal/testutils/render.go	51.11% <51.11%> (ø)
internal/jbig2/document/segments/text-region.go	41.05% <0.00%> (+0.22%)	:arrow_up:
...ernal/jbig2/document/segments/symbol-dictionary.go	38.49% <0.00%> (+0.23%)	:arrow_up:
internal/jbig2/bitmap/bitmap.go	76.76% <0.00%> (+0.28%)	:arrow_up:
model/writer.go	77.47% <0.00%> (+0.41%)	:arrow_up:
internal/jbig2/document/segments/generic-region.go	47.75% <0.00%> (+1.18%)	:arrow_up:
fdf/parser.go	40.27% <0.00%> (+1.38%)	:arrow_up:
model/reader.go	48.96% <0.00%> (+1.44%)	:arrow_up:
internal/jbig2/document/segments/kind.go	11.59% <0.00%> (+1.44%)	:arrow_up:
... and 57 more

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