tests: Portability fixes, replaced 0/1/-1 return codes with macros

tests/stdio/fwrite.c

@@ -7,18 +7,18 @@ int main(void) {
 	const char ptr[] = "Hello World!";
 
 	if (fwrite(ptr, 0, 17, f)) {
-		return -1;
+		return EXIT_FAILURE;
 	}
 
 	if (fwrite(ptr, 7, 0, f)) {
-		return -1;
+		return EXIT_FAILURE;
 	}
 
 	if (fwrite(ptr, 0, 0, f)) {
-		return -1;
+		return EXIT_FAILURE;
 	}
 
 	fwrite(ptr, sizeof(ptr), 1, f);
 	fclose(f);
-	return 0;
+	return EXIT_SUCCESS;
 }