Implement strstr(), add strstr test

tests/string/strstr.c

@@ -0,0 +1,18 @@
+#include <string.h>
+#include <stdio.h>
+
+int main(int argc, char* argv[]) {
+    // should be "rust"
+    char* str1 = strstr("In relibc we trust", "rust");
+    printf("%s\n", (str1) ? str1 : "NULL"); 
+
+    // should be "libc we trust"
+    char* str2 = strstr("In relibc we trust", "libc");
+    printf("%s\n", (str2) ? str2 : "NULL"); 
+
+    // should be "NULL"
+    char* str3 = strstr("In relibc we trust", "bugs");
+    printf("%s\n", (str3) ? str3 : "NULL"); 
+
+    return 0;
+}