Return types not being checked for consistency in test framework

Description

Somehow the test framework should be checking that return types are consistent between different autodiff implementations.

What I mean by consistent is if a function returns a matrix in prim, it returns a matrix in rev. This is not the same as returning the same type (since the scalar types will be different in the different versions).

That is not the case currently.

This test passes:

#include <test/unit/math/test_ad.hpp>

template <typename T,
          stan::require_not_var_t<T>* = nullptr>
auto myfunc(const T& x1) {
  T x2 = x1;
  return x2;
}

template <typename T,
          stan::require_var_t<T>* = nullptr>
auto myfunc(const T& x1) {
  Eigen::Matrix<T, -1, -1> x2(1, 1);
  x2 << x1;
  return x2;
}

TEST(MathMixMatFun, mat_vs_var) {
  auto f = [](const auto& x) { return myfunc(x); };

  double d = 2;
  stan::test::expect_ad(f, d);
}

Current Version:

v3.3.0

Start with test/unit/math/test_ad.hpp and the stan::test::expect_ad call shown in the reproducer. Trace how return values from prim and rev implementations are compared, then add coverage for matrix-versus-scalar return mismatches. Done means the supplied mat_vs_var example is rejected while valid autodiff return shapes remain accepted.