"Include what you use" for tests

Using strcpy/strlen in global namespace requires `<string.h>`. Include
explicitly instead of assuming another SWIG or library header brings it
in.

Examples/test-suite/director_binary_string.i

@@ -8,6 +8,7 @@
 
 %inline %{
 #include <stdlib.h>
+#include <string.h>
 
 #define BUFFER_SIZE_AA 8
 #define BUFFER_SIZE_BB 5