First integration test for mailbox with two contacts

...org/briarproject/bramble/test/BrambleIntegrationTest.java

mailbox-integration-tests/build.gradle

...oject/bramble/mailbox/AbstractMailboxIntegrationTest.java

.../briarproject/bramble/mailbox/MailboxIntegrationTest.java

...ject/bramble/mailbox/MailboxIntegrationTestComponent.java

...e/mailbox/OwnMailboxContactListWorkerIntegrationTest.java

...est/java/org/briarproject/bramble/test/FakeTorPlugin.java

...a/org/briarproject/bramble/test/FakeTorPluginFactory.java

...arproject/bramble/test/MailboxTestPluginConfigModule.java