JS/Kotlin integration tests

[gnprice]

We should have some tests that run our Kotlin code and our JS code together, for areas where they interact.

• An existing place this would be useful is for notifications: FcmMessage.kt at MessageFcmMessage#dataForOpen constructs an object which src/notification/types.js describes and src/notification/notifOpen.js consumes. That alignment is currently backed only by comments in each place pointing to the other one, and by manual testing when we edit that code.

  We don't often edit that code, though, and rarely by outside contributors, which is why that's been an acceptable state of affairs to date.

• This will become much more important with Read our stored Redux data from Kotlin #5117. With that subsystem, the interaction between JS and Kotlin would be potentially broken whenever we make changes to the schema (whenever we add a migration in src/storage/migrations.js).

  That's something we do routinely, and that all code contributors regularly do -- it's in our new-feature tutorial for that reason. So relying on carefulness and manual testing would be too unreliable and costly, and we'll need automated tests.

The way to build these tests will probably be as Android instrumented tests: the test harness will make a special build of the app, install it on an Android device (physical or emulated), and run it there. This special build has the app's code on hand, but also test code, which sits in the driver's seat.

RN itself has some tests (in react-native:ReactAndroid/src/androidTest/; see on GH, or tree node_modules/react-native/ReactAndroid/src/androidTest/) which take this form: the Java test framework is in charge, the Java test code invokes JS, and JS test code answers back. So we'll take those as a model.

[gnprice]

I spent a good bit of time on this Friday, and a bit more time today.

I still expect that this will be possible, but it's messier than I hoped. In particular, even just running RN's own tests straight from a react-native worktree — not involving our app at all — is much gnarlier than one would like, and I didn't successfully do so after spending a large chunk of Friday on it. (The next thing to try will be to use a Docker image that some folks in the community seem to maintain.)

One thing I spotted today is that those integration tests within RN use Hermes: see facebook/react-native@316b470b7 . So using those as a model will probably go most smoothly if we're also using Hermes. This prompted me to go try pushing that migration forward, aka #4131.