Add a test target and one unit test, using Check.

[schmonz]

Had a little time on the plane. Here's an extremely minimal example of a unit test. Not looking for approval yet, just feedback.

You'll need Check installed, and depending where it goes you may need to adjust conf-cc and conf-ld to find it.

Then run make test and behold, at least one use of stralloc is shown to work!

I picked stralloc because we all understand it, it's heavily used, it's relatively easy to test, and who knows, maybe with sufficient stralloc tests we'll discover something interesting we'll be glad to know about.

[schmonz]

I believe this branch is now merge-ready. It's one small test for a stralloc, one giant leap for giving us a clear way to keep adding tests.

Before building and running the tests, install Check (and pkg-config or pkgconf to find Check's includes and libs). These are dependencies for tests and only for tests.

To build and run the tests from the top-level Makefile, simply make test. Verified to work with recent BSD and GNU make.

To add more stralloc tests, copy the test_stralloc_thingy example in tests/unittest_stralloc.c. To add other tests, copy the unittest_stralloc example in tests/Makefile.

Travis and Cirrus now run make test, too.

In our too-brief meetup on Friday, @DerDakon suggested running tests via cmake to reduce build complexity, generate test reports, and possibly help in other ways. I'm open to it, but I'd love to see us first write a bunch of tests and learn what's annoying about that. :-)

The sooner we merge this, the sooner we can start adding tests with most every commit. The sooner we have existing code covered by test, the sooner we can make bolder changes. With good test coverage we can also easily get good valgrind coverage. So I'd like to see this go in ASAP, with the understanding that the way we write, run, and use the results of tests will necessarily need to be refined over time.

[DerDakon]

@schmonz agreed that I can add another testcase I was just playing with, so I have extended this by a second commit that tests additional behavior of the stralloc functions.

[schmonz]

Happy with the additions, at a glance. Please feel free to merge.