tests: Remove redundant return statements

When the execution reaches the end of the main functions, they implicitly return a successful status.

tests/string/strstr.c

@@ -1,6 +1,5 @@
 #include <string.h>
 #include <stdio.h>
-#include <stdlib.h>
 
 int main(void) {
     printf("%s\n", strstr("In relibc we trust", "rust"));
@@ -8,6 +7,4 @@ int main(void) {
     printf("%s\n", strstr("In relibc we trust", "bugs"));
     printf("%s\n", strstr("IN RELIBC WE TRUST", "rust"));
     printf("%s\n", strcasestr("IN RELIBC WE TRUST", "rust"));
-
-    return EXIT_SUCCESS;
 }