add better string representation for Process objects

[mr-c]

• add tests for all the various Process derived classes

[codecov]

Codecov Report

Merging #1571 (9195283) into main (0ad8209) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1571      +/-   ##
==========================================
+ Coverage   66.11%   66.13%   +0.02%     
==========================================
  Files          91       91              
  Lines       16309    16313       +4     
  Branches     4342     4342              
==========================================
+ Hits        10782    10789       +7     
+ Misses       4379     4376       -3     
  Partials     1148     1148              

Impacted Files	Coverage Δ
cwltool/context.py	97.29% <100.00%> (ø)
cwltool/process.py	86.08% <100.00%> (+0.04%)	⬆️
process.py	70.18% <0.00%> (-0.07%)	⬇️
cwltool/job.py	77.55% <0.00%> (+0.40%)	⬆️
cwltool/workflow.py	85.18% <0.00%> (+0.82%)	⬆️

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