Update MainDispatcherRule.kt

Updated MainDispatcherRule parameter accessibility to private because its only needs to be accessible inside that class.

Author: mariomanhique

core/testing/src/main/kotlin/com/google/samples/apps/nowinandroid/core/testing/util/MainDispatcherRule.kt

@@ -30,7 +30,7 @@ import org.junit.runner.Description
  * for the duration of the test.
  */
 class MainDispatcherRule(
-    val testDispatcher: TestDispatcher = UnconfinedTestDispatcher(),
+    private val testDispatcher: TestDispatcher = UnconfinedTestDispatcher(),
 ) : TestWatcher() {
     override fun starting(description: Description) {
         Dispatchers.setMain(testDispatcher)