test.h: Make test functions static

This makes the compiler complain about a test that was not registered with
ADDTEST.

src/test.h

@@ -149,7 +149,7 @@ must_alloc(testctx t, size_t size)
 
 #define ADDTEST(name) {#name, name##_test}
 
-#define TEST(name, t) void name##_test(testctx t)
+#define TEST(name, t) static void name##_test(testctx t)
 
 #ifdef __cplusplus
 }