[geary] Trivial test case cleanup

[Michael Gratton <mjog src gnome org>]

commit 00d381d6a99fb3fdfa175f0b05fc1d86c3967a64
Author: Michael Gratton <mike vee net>
Date:   Fri Feb 8 10:13:49 2019 +1100

    Trivial test case cleanup

 test/engine/imap-db/imap-db-account-test.vala | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/test/engine/imap-db/imap-db-account-test.vala b/test/engine/imap-db/imap-db-account-test.vala
index c6aac242..26ccf91b 100644
--- a/test/engine/imap-db/imap-db-account-test.vala
+++ b/test/engine/imap-db/imap-db-account-test.vala
@@ -60,6 +60,7 @@ class Geary.ImapDB.AccountTest : TestCase {
         );
         this.account.close_async.end(async_result());
         this.account = null;
+        this.config = null;
 
         delete_file(this.tmp_dir);
         this.tmp_dir = null;