Fix test_model51.py on Ice 3.4

Ice 3.5 generates an _enumerators collection to
access the members of an enum. Ice 3.4 generates
a _names collection of the string values of the
enums instead. For the moment, handling this only
in the tests. We will likely want to provide some
helper methods for working with these.

Note: my intent had been to inject a _enumerators
collection in the Ice 3.4 code, but there's not a
good hook for doing so.
Merge pull request #3203 from joshmoore/units-ice34

Fix test_model51.py on Ice 3.4