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

src/org/thoughtcrime/securesms/util/VisibleForTesting.java

@@ -1,4 +0,0 @@
-package org.thoughtcrime.securesms.util;
-
-public @interface VisibleForTesting {
-}