Better testing framework

[sdatkinson]

See #164.

Catch2 seems reasonable. Opening this Issue to invite discussion. cc @Khalian

[Khalian]

#164 is a bit premature. I am redoing this work, and will most likely end up with a new PR.

So Catch2 is reasonable and the reason I proposed it 3 fold

1. Setting up a testing framework with proper help functionality, test enumeration etc.
2. Natively supporting benchmarking idiomatically.
3. Setup a way to check for memory leaks (e.g. using valgrind) against the tests on catch2.

Catch2 happens to be the simplest way to do C++ tests and benchmarking idiomatically

#166 solves the problem properly.