[DI] Make backing field of TestDispatcher + scheduler private

app/src/androidTest/kotlin/at/bitfire/davdroid/HiltTestRunner.kt

@@ -12,9 +12,6 @@ import androidx.test.runner.AndroidJUnitRunner
 import at.bitfire.davdroid.di.TestCoroutineDispatchersModule
 import at.bitfire.davdroid.sync.SyncAdapterService
 import dagger.hilt.android.testing.HiltTestApplication
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.ExperimentalCoroutinesApi
-import kotlinx.coroutines.test.setMain
 
 @Suppress("unused")
 class HiltTestRunner : AndroidJUnitRunner() {
@@ -22,7 +19,6 @@ class HiltTestRunner : AndroidJUnitRunner() {
     override fun newApplication(cl: ClassLoader, name: String, context: Context): Application =
         super.newApplication(cl, HiltTestApplication::class.java.name, context)
 
-    @OptIn(ExperimentalCoroutinesApi::class)
     override fun onCreate(arguments: Bundle?) {
         super.onCreate(arguments)
 
@@ -34,7 +30,7 @@ class HiltTestRunner : AndroidJUnitRunner() {
         SyncAdapterService.syncActive.set(false)
 
         // set main dispatcher for tests (especially runTest)
-        Dispatchers.setMain(TestCoroutineDispatchersModule.standardTestDispatcher)
+        TestCoroutineDispatchersModule.initMainDispatcher()
     }
 
 }

app/src/androidTest/kotlin/at/bitfire/davdroid/di/TestCoroutineDispatchersModule.kt

@@ -10,8 +10,10 @@ import dagger.Provides
 import dagger.hilt.components.SingletonComponent
 import dagger.hilt.testing.TestInstallIn
 import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.test.StandardTestDispatcher
-import kotlinx.coroutines.test.TestCoroutineScheduler
+import kotlinx.coroutines.test.setMain
 
 /**
  * Provides test dispatchers to be injected instead of the normal ones.
@@ -26,8 +28,7 @@ import kotlinx.coroutines.test.TestCoroutineScheduler
 )
 object TestCoroutineDispatchersModule {
 
-    val scheduler = TestCoroutineScheduler()
-    val standardTestDispatcher = StandardTestDispatcher(scheduler)
+    private val standardTestDispatcher = StandardTestDispatcher()
 
     @Provides
     @DefaultDispatcher
@@ -45,4 +46,14 @@ object TestCoroutineDispatchersModule {
     @SyncDispatcher
     fun syncDispatcher(): CoroutineDispatcher = standardTestDispatcher
 
+   /**
+     * Sets the [standardTestDispatcher] as [Dispatchers.Main] so that test dispatchers
+     * created in the future use the same scheduler. See [StandardTestDispatcher] docs
+     * for more information.
+     */
+    @OptIn(ExperimentalCoroutinesApi::class)
+    fun initMainDispatcher() {
+        Dispatchers.setMain(standardTestDispatcher)
+    }
+
 }