[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/string_simple.i

@@ -1,10 +1,10 @@
 %module string_simple
 
-%newobject copy_string;
+%newobject copy_str;
 
 %inline %{
 #include <string.h>
-const char* copy_string(const char* str) {
+const char* copy_str(const char* str) {
   size_t len = strlen(str);
   char* newstring = (char*) malloc(len + 1);
   strcpy(newstring, str);