Final round of refactoring of regression framework

[lintool]

Final installment of #1680

• Refactored regression script, cleaned up dead code.
• Renamed YAML params.

[codecov-commenter]

Codecov Report

Merging #1717 (f967e72) into master (6500560) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1717   +/-   ##
=========================================
  Coverage     57.58%   57.58%           
  Complexity     1032     1032           
=========================================
  Files           173      173           
  Lines          9781     9781           
  Branches       1344     1344           
=========================================
  Hits           5632     5632           
  Misses         3675     3675           
  Partials        474      474           

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[lintool]

@MXueguang This is ready for review.

I've confirmed that regressions still work; most of the changes are cosmetic, so a quick sanity check will suffice.

Most of the substantive changes are in the regression script src/main/python/run_regression.py.