Add a Doxyfile and preliminary module-level docs.

src/tests.h

@@ -1,5 +1,11 @@
 #pragma once
 
+/**
+** \defgroup tests Tests
+** This is the test harness.
+** @{
+*/
+
 typedef struct _tf_test_options_t
 {
 	const char* exe_path;
@@ -7,3 +13,5 @@ typedef struct _tf_test_options_t
 } tf_test_options_t;
 
 void tf_tests(const tf_test_options_t* options);
+
+/** @} */