Use gtest for testing

[leonardt]

I setup gtest to handle testing https://github.com/leonardt/verilogAST-cpp, it was really simple to integrate into a cmake based workflow.

Here's some references I used:

• https://github.com/google/googletest/blob/master/googletest/docs/primer.md
• https://github.com/google/googletest/blob/master/googletest/README.md#incorporating-into-an-existing-cmake-project

Here are some example files to work off of:

• https://github.com/leonardt/verilogAST-cpp/blob/master/tests/basic.cpp
• https://github.com/leonardt/verilogAST-cpp/blob/master/CMakeLists.txt#L43-L45
• https://github.com/leonardt/verilogAST-cpp/blob/master/.travis.yml

I would vote that if someone wants to improve the testing infrastructure for coreir that we use a similar approach, but I'm also open to other frameworks. https://github.com/catchorg/Catch2 is a simpler, lighterweight option that I considered. It looks like the simulator is already using this, so that's another viable option that I would support.

[rsetaluri]

Huge fan of gtest and fairly experienced with it so, I'm in favor of running with it.