Fix stale usage of emptyObject in cloneWithProps-test.

After we run `require('mock-modules').dumpCache()`, the object exported by
the `emptyObject` module will no longer be identical to previously
exported objects, so tests like `expect(component.refs).toBe(emptyObject)`
will fail.

Note that this behavior only manifests itself in tests, because of course
we do not call `dumpCache` in production code.

We could consider storing the `emptyObject` globally to thwart the effects
of `dumpCache`, but it's more idiomatic simply to re-`require` the latest
version of `emptyObject`.
Re-require `cloneWithProps` as well, for consistency.

And don't mock `emptyObject`, since that might actually replace it with a
mutable/non-frozen object.
Merge pull request #1215 from benjamn/fix-cloneWithProps-test

Fix stale usage of emptyObject in cloneWithProps-test.