Additional tests for complex overloads

[WardBrian]

Summary

PR #2753 allowed complex numbers to work with the SVD family of functions. Before exposing them to stanc, I decided to add extra tests. Truth values taken from numpy

Tests

Adds tests to singular_values, svd_U, and svd_V for a complex matrix input.

Side Effects

Added EXPECT_MATRIX_COMPLEX_FLOAT_EQ to test equality of complex-valued matrices.

Release notes

Checklist

• Math issue : Related to Complex support for more linear algebra functions #2699

• 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

[rok-cesnovar]

Looks good to me.