Use a proper unit-testing system framework

[lewissbaker]

The tests for cppcoro are currently written using plain functions and standard library asserts.
While these are functional, it would be nice to make use of a system that eliminated the boiler-plate code, made it easier to split tests across different source files, provided better reporting of tests and failures, command-line options for running individual tests.

[lewissbaker]

I have a unit-test framework I've written for a side-project that provides much of this functionality that I could use but I do wonder whether something like the googletest might be more appropriate for an open-source project.

[lewissbaker]

Another framework worth looking into is https://github.com/onqtam/doctest

[lewissbaker]

Changed to use the doctest framework and split tests across multiple files in 9a620aa.

The doctest framework does potentially have some issues with running checks on multiple threads so it may or may not prove to be suitable for task.