i3test: add kill_all_windows convenience function

testcases/lib/i3test.pm.in

@@ -46,6 +46,7 @@ our @EXPORT = qw(
     wait_for_map
     wait_for_unmap
     $x
+    kill_all_windows
 );
 
 =head1 NAME
@@ -899,6 +900,17 @@ sub get_i3_log {
     return slurp($logfile);
 }
 
+=head2 kill_all_windows
+
+Kills all windows to clean up between tests.
+
+=cut
+sub kill_all_windows {
+    # Sync in case not all windows are managed by i3 just yet.
+    sync_with_i3;
+    cmd '[title=".*"] kill';
+}
+
 =head1 AUTHOR
 
 Michael Stapelberg <michael@i3wm.org>