grader: always run pretest_test_cases if available

[Xyene]

Ref #1068.

[codecov-commenter]

Codecov Report

Base: 80.72% // Head: 83.61% // Increases project coverage by +2.89% 🎉

Coverage data is based on head (2252329) compared to base (bececf2).
Patch coverage: 58.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1070      +/-   ##
==========================================
+ Coverage   80.72%   83.61%   +2.89%     
==========================================
  Files         134      134              
  Lines        4747     4755       +8     
==========================================
+ Hits         3832     3976     +144     
+ Misses        915      779     -136     

Impacted Files	Coverage Δ
dmoj/graders/base.py	78.04% <54.54%> (-9.84%)	⬇️
dmoj/judge.py	54.54% <100.00%> (+1.21%)	⬆️
dmoj/executors/java_executor.py	84.84% <0.00%> (+2.02%)	⬆️
dmoj/cptbox/compiler_isolate.py	55.31% <0.00%> (+6.38%)	⬆️
dmoj/cptbox/tracer.py	76.32% <0.00%> (+18.02%)	⬆️
dmoj/cptbox/handlers.py	100.00% <0.00%> (+26.31%)	⬆️
dmoj/cptbox/isolate.py	89.63% <0.00%> (+38.41%)	⬆️
dmoj/control.py	100.00% <0.00%> (+64.70%)	⬆️

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