Reorganize test code a bit

Reorganize test code a bit

A few guiding principles:
* Each test source file includes ONLY "test.h" to make it easy
  to create new tests.
* Each test suite has a "test.h" that includes "test_util/test_common.h"
  to get access to all the common testing utility functions.

So "test_common.h" is then responsible for including
any smaller headers that declare specific pieces of
shared test functionality.

I've also pulled some test filtering logic that was _only_ used
in test_main.c into that file, and repurposed "test_utils.[ch]"
for common utility code.  (Eventually, a lot of the assertion
helpers currently in "test_main.c" should probably be organized
into one or more source files of their own.)