printf_unit_test: Disable Wformat-truncation on GCC >= 7

We intentionally pass NULL as argument to format, hence disable the warning.

tests/printf_unit_test.c

@@ -23,6 +23,10 @@
 
 #define MSG "printf test failed: %s, %s", real, expected
 
+#if defined(__GNUC__) && __GNUC__ >= 7
+#pragma GCC diagnostic ignored "-Wformat-truncation="
+#endif
+
 static void
 printf_test (void)
 {