Slow down the reveal effect a little on background changes

shell/plugins/background/Background.qml

@@ -274,7 +274,7 @@ Item {
     property: "revealProgress"
     from: 0
     to: 1
-    duration: Style.duration(420)
+    duration: Style.duration(840)
     easing.type: Easing.OutCubic
     onFinished: {
       if (root.incomingBackground) {

test/shell.d/background-test.sh

@@ -74,4 +74,9 @@ assert(
     /function prepareBackground[\s\S]*?requestNativeSize\(path\)/.test(backgroundQml),
   'background never probes videos and probes a prepared frame ahead of its transition'
 )
+
+assert(
+  /id: revealAnimation[\s\S]*?duration: Style\.duration\(840\)/.test(backgroundQml),
+  'background reveal takes 840ms'
+)
 JS