tests: Replace returns with exits in the main functions

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

tests/stdio/fwrite.c

@@ -7,15 +7,15 @@ int main(void) {
 	const char ptr[] = "Hello World!";
 
 	if (fwrite(ptr, 0, 17, f)) {
-		return EXIT_FAILURE;
+		exit(EXIT_FAILURE);
 	}
 
 	if (fwrite(ptr, 7, 0, f)) {
-		return EXIT_FAILURE;
+		exit(EXIT_FAILURE);
 	}
 
 	if (fwrite(ptr, 0, 0, f)) {
-		return EXIT_FAILURE;
+		exit(EXIT_FAILURE);
 	}
 
 	fwrite(ptr, sizeof(ptr), 1, f);