Fix expression tests for new Wiener functions

[WardBrian]

Summary

This unblocks the new wiener cdf functions from #3042 for inclusion in the language (https://github.com/stan-dev/stanc3/pull/1576).

1. Adds missing precision default.
2. Cleans up an unused variable warning.
3. Updates our expression test generation code to correctly generate test cases for these new functions
4. Updates some of the ref logic to avoid double-evaluation of expression templates (same as #3112)

Tests

Expression tests now passing.

Side Effects

None

Checklist

• [x] Copyright holder: Simons Foundation

  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/)

• [x] 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)

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

• [x] the new changes are tested