Move tests to a separate repo

[qwertymodo]

The test directory is very large (currently >150MB), and checking out the repo requires checking out that entire test directory as well, even if you just want to build the code without running the tests. Splitting the test directory into a separate repo and including it as a submodule or subtree would eliminate this issue and make it a lot more friendly for people to use this library from the git repo.

[nlohmann]

Duplicate of #96

Discussion on a solution: #1184