Run the example app tests in CI

[rfk]

In https://github.com/mozilla/uniffi-rs/issues/987 we identified an issue that is causing build failures in the iOS sample app, and which would cause build failures for our application-services iOS build in a production release. I happened to discover this accidentally, because it's not at all obvious that one should build and test the sample app as part of our usual development flow.

Let's guard against this in future by running the sample app tests (or at the very least, building it) in CI. Unfortunately this may require the use of more expensive osx machines rather than the cheaper linux docker ones that we're currently using.

If that proves too hard or too expensive, we could also add "manually check that the sample app builds and runs" to the release-cutting checklist.

┆Issue is synchronized with this Jiraserver Task ┆Issue Number: UNIFFI-74