test: Break out engine mock objects into their own name-space

Engine mocks don't need to be in the `Geary` namespace, and including
them there makes it difficult to use them in client tests, so put them
all in their own name-space and corresponding directory.

test/engine/imap-engine/imap-engine-generic-account-test.vala

@@ -48,7 +48,7 @@ public class Geary.ImapEngine.GenericAccountTest : TestCase {
         this.config = new Geary.AccountInformation(
             "test",
             ServiceProvider.OTHER,
-            new MockCredentialsMediator(),
+            new Mock.CredentialsMediator(),
             new Geary.RFC822.MailboxAddress(null, "test@example.com")
         );