UnicodePlots: adjust layout width per column

[t-bltg]

• Save some width (previously based on the largest column);
• Add more supported attributes;
• Move expensive tests (GR) at the end of the test suite.

[codecov]

Codecov Report

Merging #3825 (543bc66) into master (8573993) will increase coverage by 0.55%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master    #3825      +/-   ##
==========================================
+ Coverage   63.01%   63.56%   +0.55%     
==========================================
  Files          28       28              
  Lines        7467     7463       -4     
==========================================
+ Hits         4705     4744      +39     
+ Misses       2762     2719      -43     

Impacted Files	Coverage Δ
src/backends.jl	63.47% <ø> (ø)
src/backends/unicodeplots.jl	75.00% <81.25%> (+33.59%)	⬆️
src/plot.jl	66.20% <0.00%> (-4.83%)	⬇️
src/recipes.jl	59.71% <0.00%> (+0.21%)	⬆️
src/axes.jl	85.16% <0.00%> (+0.43%)	⬆️
src/output.jl	28.68% <0.00%> (+0.81%)	⬆️
src/colorbars.jl	92.72% <0.00%> (+1.81%)	⬆️

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