[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/Makefile.in

@@ -35,7 +35,6 @@ director_using \
 enum_thorough \
 li_windows \
 member_pointer_const \
-minherit \
 nested_directors \
 preproc_constants \
 smart_pointer_inherit \
@@ -45,8 +44,6 @@ using_protected \
 FAILING_C_TESTS = \
 enums \
 preproc_constants_c \
-string_simple \
-unions \
 
 ml_runme = $(SCRIPTPREFIX)$*$(SCRIPTSUFFIX)