[google_maps_flutter] fix android tests

packages/google_maps_flutter/google_maps_flutter_android/android/src/test/java/io/flutter/plugins/googlemaps/MarkersControllerTest.java

@@ -10,6 +10,7 @@
 import static org.mockito.Mockito.when;
 
 import android.content.res.AssetManager;
+import android.os.Build;
 import androidx.test.core.app.ApplicationProvider;
 import com.google.android.gms.maps.GoogleMap;
 import com.google.android.gms.maps.model.LatLng;
@@ -25,14 +26,21 @@
 import java.util.Map;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
 import org.mockito.Mockito;
+import org.mockito.MockitoAnnotations;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.annotation.Config;
 
+@RunWith(RobolectricTestRunner.class)
+@Config(sdk = Build.VERSION_CODES.P)
 public class MarkersControllerTest {
   private AssetManager assetManager;
   private final float density = 1;
 
   @Before
   public void before() {
+    MockitoAnnotations.openMocks(this);
     assetManager = ApplicationProvider.getApplicationContext().getAssets();
   }