test: split plot tests into finer-scoped files

[wpbonelli]

• split test_plot.py into separate files for different plotting classes / use cases
• no tests added/removed, just reorganized
• moved from (and motivated by) Add PRT support #1753
  • with PRT support comes more plotting-related tests and test_plot.py was growing unwieldy

[codecov]

Codecov Report

Merging #1949 (b36915a) into develop (9e9d730) will increase coverage by 0.0%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop   #1949     +/-   ##
=========================================
  Coverage     72.6%   72.6%             
=========================================
  Files          257     257             
  Lines        57812   56437   -1375     
=========================================
- Hits         42009   41028    -981     
+ Misses       15803   15409    -394     

see 75 files with indirect coverage changes