Unit testing

[azonenberg]

Top level meta-issue for setting up some kind of unit test framework. CTest may be worth using since it integrates well with CMake, but I'm open to other options.

The hard part of course is going to be figuring out what to test, refactoring code to be more testable, etc.

[azonenberg]

There's now a simple unit test for FrequencyMeasurement in #242 (tests/FrequencyMeasurement). It runs with no arguments and exits with 0 on success and nonzero on failure.

[tarunik]

Catch2 is probably the simplest to get started with, at least in its current stable incarnation -- testcases are just funny looking free functions, with setup/teardown handled elegantly, and very little configuration needed, while still supporting rich assertion output.

[azonenberg]

Proposal:

• CTest as top level test framework
• Use CTest's "one executable per test case" model for testing filters as they tend to be more complex and have more baggage
• Use Catch2 under CTest for smaller scale testing of single functions etc