"Include what you use" for tests

Many of these tests implicitly required the target language library files to include the headers upstream.

Examples/test-suite/string_simple.i

@@ -3,6 +3,7 @@
 %newobject copy_string;
 
 %inline %{
+#include <stdlib.h>
 #include <string.h>
 const char* copy_string(const char* str) {
   size_t len = strlen(str);