[OCaml] Fix and re-enable the string_simple, minherit, and unions tests

copy_string() is a macro in the OCaml C API, so rename the function
to copy_str(). Add a runtime test.

The minherit runtime test was fixed by b64d685.

Use the proper syntax for accessing member variables in
unions_runme.ml

Examples/test-suite/ocaml/string_simple_runme.ml

@@ -0,0 +1,9 @@
+open Swig
+open String_simple
+
+let str = "olé"
+
+let copy = _copy_str (C_string str) as string
+
+let _ = if str <> copy then
+  raise (Failure ("Error: copy is not equal: original="^str^", copy="^copy))