tests: Add formatter test

tests/formatting/in2.blp

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+using Gtk 4.0;
+
+    using Adw 1;
+
+Overlay {
+
+Label
+label
+{
+label
+:
+_
+(
+"'Hello World!' \"\n\t\""
+)
+;
+}
+[
+    overlay
+] Button
+{ notify
+:: icon-name
+=> $ on_icon_name_changed ( label )
+swapped ;
+styles
+[ "destructive" ]
+}
+visible
+: bind $ isVisible ( label.visible ,
+my-menu ) as
+ < bool > ; width-request : bind label . width-request no-sync-create ; }
+                menu my-menu
+{ item ( _ ( "Label" ) , "action-name" , "icon-name" ) item { action : "win.format" ; } }
+
+
+
+
+
+
+