update UnicodePlots to 3.0.0 - coverage

[t-bltg]

• eval ref imgs in a specific module (avoids leaking);
• rename test files, clarify test structure;
• update compat entries;
• lots of simplifications;
• improve coverage.

[codecov]

Codecov Report

Merging #4250 (75c0682) into master (03d015c) will increase coverage by 1.05%.
The diff coverage is 63.35%.

@@            Coverage Diff             @@
##           master    #4250      +/-   ##
==========================================
+ Coverage   77.87%   78.93%   +1.05%     
==========================================
  Files          28       29       +1     
  Lines        7111     7041      -70     
==========================================
+ Hits         5538     5558      +20     
+ Misses       1573     1483      -90     

Impacted Files	Coverage Δ
src/backends/unicodeplots.jl	85.48% <0.00%> (+0.90%)	⬆️
src/components.jl	88.06% <0.00%> (+0.24%)	⬆️
src/utils.jl	63.94% <0.00%> (+1.72%)	⬆️
src/animation.jl	78.88% <33.33%> (+0.39%)	⬆️
src/plotmeasures.jl	40.00% <40.00%> (ø)
src/colorbars.jl	87.69% <50.00%> (+1.12%)	⬆️
src/recipes.jl	66.33% <50.00%> (+1.97%)	⬆️
src/axes.jl	85.77% <66.66%> (+0.34%)	⬆️
src/output.jl	62.92% <66.66%> (+0.06%)	⬆️
src/backends/pgfplotsx.jl	70.86% <67.74%> (-0.08%)	⬇️
... and 18 more

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