[OCaml] Runtime tests for template_opaque, using_extend and some others

Add runtime tests for namespace_virtual_method, template_opaque,
using_extend, using_inherit, using_pointers, using_private, and
varargs_overload.

Examples/test-suite/ocaml/using_private_runme.ml

@@ -0,0 +1,10 @@
+open Swig
+open Using_private
+
+let _ =
+  let f = new_FooBar '() in
+  let _ = f -> "[x]" (3) in
+  assert (f -> blah (4) as int = 4);
+  assert (f -> defaulted () as int = -1);
+  assert (f -> defaulted (222) as int = 222)
+;;