Fix chat overlay tests

Not entirely sure why they were failing previously, but the most likely
explanation is that by freak accident some mock requests would
previously execute immediately rather than be scheduled on the API
thread, which would change execution ordering and ensure that
`ChannelManager.CurrentChannel` would become the joined channel, rather
than remaining at the channel listing.

osu.Game.Tests/Visual/Online/TestSceneChatOverlay.cs

@@ -446,7 +446,7 @@ namespace osu.Game.Tests.Visual.Online
         {
             AddStep("Show overlay with channel 1", () =>
             {
-                channelManager.JoinChannel(testChannel1);
+                channelManager.CurrentChannel.Value = channelManager.JoinChannel(testChannel1);
                 chatOverlay.Show();
             });
             waitForChannel1Visible();
@@ -462,7 +462,7 @@ namespace osu.Game.Tests.Visual.Online
         {
             AddStep("Show overlay with channel 1", () =>
             {
-                channelManager.JoinChannel(testChannel1);
+                channelManager.CurrentChannel.Value = channelManager.JoinChannel(testChannel1);
                 chatOverlay.Show();
             });
             waitForChannel1Visible();