Break suite tests into more readable chunks.

Break "Test Suite Allocation," test 0 - 2 into more manageable chunks,
with shared named code sections of common variables and expressions.
Remove a few extraneous assertions and added a return to a non-void
function. Clean up some variables left around that are no longer in
use. Rename a couple of documentation sections to more clearly
illustrate intent. Added wp_test_execute_tests declaration to the
wp_test.h header.