appends name of ad types used to ad test to failure output

[SteveBronder]

Summary

When a test fails in the ad testing framework I always forget that the left hand side is the finite difference result and the right hand side is the autodiff result. This PR just adds a little line at the end of the test failure message for expect_near_rel that says what was used to get the gradients. An example is below where the error message is the same but has a little append for for {lhs_type} vs {rhs_type}.

./test/unit/math/expect_near_rel.hpp:33: Failure
The difference between x1 and x2 is 1, which exceeds tol_val, where
x1 evaluates to 1,
x2 evaluates to 2, and
tol_val evaluates to 0.00015000000000000001.
expect_near_rel_finite in: expect_near_rel; require items x1(0) = x2(0): hessian() grad for finite_diff vs fvar<var>

For a full example see the gist here for an example where I cause the add tests to fail.

Tests

No new tests since this just adds some words to the test message.

Side Effects

Nope

Release notes

Add helpful context to the failure message for the ad testing framework

Checklist

• 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/)

• 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]

Very useful!

[syclik]

Awesome!