Make pretest_test_cases and test_cases disjoint

[Xyene]

Refs DMOJ/judge-server#1068

[codecov-commenter]

Codecov Report

Base: 46.50% // Head: 46.48% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (01037ce) compared to base (b676a49).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2025      +/-   ##
==========================================
- Coverage   46.50%   46.48%   -0.03%     
==========================================
  Files         236      236              
  Lines       13104    13089      -15     
==========================================
- Hits         6094     6084      -10     
+ Misses       7010     7005       -5     

Impacted Files	Coverage Δ
judge/utils/problem_data.py	15.24% <0.00%> (-0.39%)	⬇️
django_2_2_pymysql_patch.py	42.85% <0.00%> (-42.86%)	⬇️
judge/models/submission.py	88.81% <0.00%> (-1.69%)	⬇️
judge/comments.py	39.17% <0.00%> (-1.69%)	⬇️
judge/widgets/select2.py	55.68% <0.00%> (-0.99%)	⬇️
judge/views/organization.py	38.27% <0.00%> (-0.23%)	⬇️
dmoj/settings.py	97.68% <0.00%> (ø)
judge/models/__init__.py	100.00% <0.00%> (ø)
judge/views/api/api_v2.py	47.79% <0.00%> (ø)
judge/views/submission.py	32.21% <0.00%> (ø)
... and 5 more

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