Allow stand alone test files

[nandgate]

Allow a test_foo.c file to be run without requiring an application foo.c file. This would allow tests for application headers (macros, etc.). At the moment the tests for headers need to ride along with other tests, even though the header may be used by multiple application files. This makes it hard to find and manage these tests.

[nandgate]

This would requiring updating the test runner which is already a bit magical. Keeping this open because it has utility, but for now use a empty foo.c to satisfy the test runner idioms.