[geary/wip/730682-refine-convo-list] Fix test build.

[Michael Gratton <mjog src gnome org>]

commit 695436b0fc8445f2d2d8719f7208d3b0ce36aeb8
Author: Michael James Gratton <mike vee net>
Date:   Sat Dec 23 16:57:06 2017 +1030

    Fix test build.

 test/engine/api/geary-account-test.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/test/engine/api/geary-account-test.vala b/test/engine/api/geary-account-test.vala
index 5b9e29c..2ddd799 100644
--- a/test/engine/api/geary-account-test.vala
+++ b/test/engine/api/geary-account-test.vala
@@ -121,7 +121,7 @@ public class Geary.MockAccount : Account {
         throw new EngineError.UNSUPPORTED("Mock method");
     }
 
-    public override async Gee.MultiMap<EmailIdentifier, FolderPath>
+    public override async Gee.MultiMap<EmailIdentifier, FolderPath>?
         get_containing_folders_async(Gee.Collection<EmailIdentifier> ids,
                                      Cancellable? cancellable) throws Error {
         throw new EngineError.UNSUPPORTED("Mock method");