Refactor unit tests to use the tinytest framework.

"Tinytest" is a minimalist C unit testing framework I wrote for
Libevent.  It supports some generally useful features, like being able
to run separate unit tests in their own processes.

I tried to do the refactoring to change test.c as little as possible.
Thus, we mostly don't call the tinytest macros directly.  Instead, the
test.h header is now a wrapper on tinytest.h to make our existing
test_foo() macros work.

The next step(s) here will be:
  - To break test.c into separate files, each with its own test group.
  - To look into which things we can test
  - To refactor the more fiddly tests to use the tinytest macros
    directly and/or run forked.
  - To see about writing unit tests for things we couldn't previously
    test without forking.