Clean test code

[bendk]

As part of the seperate bindings crate work, I moved some test fixtures/functions around. While doing that it occurred to me that we should clean up some of our tests:

• Merge ext-types and external-types into 1 fixture
• Do we need both a callbacks example and a callbacks fixture? If so, maybe rename the one of them, since it's kind of awkward that we have both test_callbacks.kts and test_callbacks_example.kts scripts
• Consider removing some of the regressions tests, or refactor them to work in a world where bindings crates are separate from uniffi-bindgen since some of the tests seem specific to one language.

┆Issue is synchronized with this Jira Task ┆friendlyId: UNIFFI-151