Testing: Prioritize changed unit tests

[WardBrian]

Summary

This adds a short round of testing which is run in parallel with the header checks. It sees what files are changed relative to develop and runs only the tests which either changed, or the tests of the functions which changed, if it can determine them.

The goal here is to make the math CI pipeline fail faster and with a more localized error for simple PRs. Obviously if you change a function which other functions depend on it might still fail in later stages, but this should help with iteration.

Note that these tests do get re-run later during the stage they normally would.

Other notes: OpenCL tests are excluded, and if more than 20 tests have changed we skip this to avoid it slowing down the whole CI pipeline - at that point, you probably want to move on to the later full stages anyway.

Thanks to @SteveBronder for the idea

Checklist

• Math issue #(issue number)

• Copyright holder: (fill in copyright holder information)

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the 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/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[WardBrian]

Thanks to some debugging with @serban-nicusor-toptal this is now working. See: https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FMath/detail/PR-2832/17/pipeline/165/

It correctly ran both the modified test abs_test, as well as the prim AND mix tests for asinh

[WardBrian]

@rok-cesnovar This is ready for review now, got the merge conflicts fixed up after the other CI changes

[syclik]

LGTM! Awesome. Thanks!