expand ui test suite

[nikomatsakis]

Currently we are rather lacking in "unit tests" (or rather, integration tests that are narrowly targeted). This issue tracks a checklist of arrays where we could use more targeted ui tests:

• [ ] signature of java functions matches the declaration in the Rust code
• [ ] transitive upcasts: do we generate the right lists, given A extends B extends C extends D, can we upcast directly from A to C or D?
• [ ] overloaded or duplicate methods generate some kind of error
• [ ] #80
• [ ] tests for fields (there are tests in the viper. test)
• [ ] tests for enum derive with complicated inheritance, check that we test in the right order (see this Zulip comment for some more details)
• [ ] tests for a helper function that creates and returns a impl IntoJava like this one; that function is a good test, but it is kind of a "helper function" to other tests, it'd be nice to have a dedicated UI test for it

[bowemi]

https://github.com/duchess-rs/duchess/pull/169

Adds some testing for the first checkbox