docs/howto/testing: Add/tweak for platform-specific testing.

docs/howto/testing.md

@@ -18,6 +18,16 @@ To write a test, place a Javascript file with the `-test.js` suffix in the
 `__tests__` directory inside of any subfolder of `/src`. The test will be
 automatically picked up by the test runner.
 
+We've configured Jest to run two different
+["projects"](https://jestjs.io/docs/configuration#projects-arraystring--projectconfig):
+one with an iOS preset, and one with an Android preset.
+`tools/test jest` will run one or the other, or both, depending on
+what you pass for `--platform`.
+
+If it makes sense for a test file to be dedicated to just one
+platform, and that's unlikely to change, you can instead suffix it
+with `-test.ios.js` or `-test.android.js`, and it will be ignored when
+the other platform's project runs.
 
 ### Test style guide
 
@@ -52,7 +62,9 @@ automatically picked up by the test runner.
   be mutated and hence will eventually fail tests in case of mutation.
 
 
-## Unit tests: Android
+## Native-code tests:
+
+### Android
 
 We have a small, nascent suite of unit tests for our Android-native
 (Kotlin and Java) code.
@@ -88,6 +100,12 @@ Other sources which might be helpful to read or refer to:
 [baeldung-junit-kotlin]: https://www.baeldung.com/junit-5-kotlin
 [baeldung-truth]: https://www.baeldung.com/google-truth
 
+### iOS
+
+We don't yet have native-code tests for iOS. We should try it out! A
+good incremental step will be to at least check that the build
+completes without errors.
+
 
 ## Functional tests
 

tools/test

@@ -157,7 +157,7 @@ run_native() {
     else
         :
         # TODO: uncomment when we actually have iOS native tests, like
-        # making a build.
+        # making a build. Also update docs/howto/testing.md.
         # echo -e $'\e[31mSkipping native iOS tests. Use --platform to control this.\e[0m';
     fi
 }