Add a bunch of edge-case tests

[WardBrian]

This adds tests for various edge cases in the compiler which were not previously tested.

These were found via mutation testing using a package called mutaml.

In addition to more tests, this PR also removes some dead code found by this method, and fixes a small bug in the typechecker error messaging for lupdf functions caused by a typo. I also made the test runner log the exit code of the tests into the output files, which is the majority of the diff here.

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.82%. Comparing base (1931ba6) to head (26acd24).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/frontend/Typechecker.ml	87.50%	1 Missing ⚠️
src/stan_math_backend/Lower_stmt.ml	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1456      +/-   ##
==========================================
+ Coverage   89.49%   89.82%   +0.32%     
==========================================
  Files          63       63              
  Lines       10618    10598      -20     
==========================================
+ Hits         9503     9520      +17     
+ Misses       1115     1078      -37     

Files with missing lines	Coverage Δ
src/frontend/Deprecation_analysis.ml	80.00% <ø> (ø)
src/frontend/Preprocessor.ml	100.00% <100.00%> (+2.66%)	⬆️
src/middle/UnsizedType.ml	82.01% <100.00%> (+1.80%)	⬆️
src/middle/Utils.ml	86.48% <ø> (-1.61%)	⬇️
src/frontend/Typechecker.ml	92.95% <87.50%> (+1.17%)	⬆️
src/stan_math_backend/Lower_stmt.ml	95.65% <0.00%> (ø)

... and 8 files with indirect coverage changes