Add test bootstrap.

With a lot of the test framework fleshed out already, I went back and
began adding tests to test the test framework. This is backwards, but it
is what it is. I'll slowly add tests until I feel confident in the test
framework, but I'm satisfied that it's ready to merge into master at
this point.

 - Add additional documentation.

 - Add test functions to some test suite functions. They're currently
   "in-line" with one another, though this may change.

 - Add bootstrap and test harness utilities, such as wp_test_assert.

 - Clean up test registration method with WP_TEST_REGISTER macro

 - Add utility macro WP_TEST_BEGIN and WP_TEST_END (yuck!) but it's just
   to get the tests bootstrapped; I'll work out something nice, later.

 - Incorporate assertions.

 - Reorganized code, but just so.

 - Fix silly linked-list issue in wp_test_suite_register.