Don't permanently modify user's make/local in test suite

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

test-model-compile.R added PRECOMPILED_HEADERS=false to make/local on every test run and never removed it.

This PR adds a local_cmdstan_make_local() test helper that writes cpp_options to make/local and restores its original contents (or absence) at the end of the test file.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.79%. Comparing base (4a3ddb8) to head (fb0539c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1221   +/-   ##
=======================================
  Coverage   91.79%   91.79%           
=======================================
  Files          15       15           
  Lines        6249     6249           
=======================================
  Hits         5736     5736           
  Misses        513      513           

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