Get the automated tests up and running again

* Fix the build.gradle file
* Use the correct VisibleForTesting class
* Fix the Android test setUp() logic
* Enable a test in DeliveryReceiptJobTest.java where the @Test anotation was missing

It should be noted that the tests in AttachmentDatabaseTest.java fail.

Fixes #3474
Closes #5882

test/unitTest/java/org/thoughtcrime/securesms/jobs/DeliveryReceiptJobTest.java

@@ -45,6 +45,7 @@ public class DeliveryReceiptJobTest extends BaseUnitTest {
     assertTrue(captor.getValue().getNumber().equals("+14152222222"));
   }
 
+  @Test
   public void testNetworkError() throws IOException {
     SignalServiceMessageSender textSecureMessageSender = mock(SignalServiceMessageSender.class);
     long                    timestamp               = System.currentTimeMillis();