"Cannot mix test roots with non-test roots"

I am trying to run instrumented tests for an Android app, but get this compile-time error:

Cannot mix test roots with non-test roots:
    Non-Test Roots: com.my.application.MyApplication
    Test Roots: [com.my.test.ActivityTest]

The application uses a custom app annotated with @HiltAndroidApp, since the docs imply that this is required for Hilt.

My test follows this pattern:

@RunWith(AndroidJUnit4.class)
@HiltAndroidTest
class MyTest {
  @Rule
  public HiltAndroidRule hiltRule = new HiltAndroidRule(this);

  @Inject
  SomeStuff stuff;

  @Before
  public void setUp() {
    hiltRule.inject();
  }

The app runner is creating a normal Hilt app:

public class MyAppRunner extends AndroidJUnitRunner {

  @Override
  public Application newApplication(ClassLoader cl, String className, Context context)
      throws InstantiationException, IllegalAccessException, ClassNotFoundException {
        return super.newApplication(
            cl, HiltTestApplication.class.getName(), context);

When running a test, I get the compile-time error outlined above. I suppose that the reference to MyApplication comes from the app's AndroidManifest.xml file.

I tried using a @CustomTestApplication with MyApplication, but custom test applications apparently don't work with applications annotated with @HiltAndroidApp.

What is the proper way to set up a test with a Hilt-based application?

from "Cannot mix test roots with non-test roots"