t/130-close-empty-split: clean up old workspace

This hopefully increases the reliability of the test. Empty containers
created by “open” sometimes get spurious UnmapNotify events, and in at
least one run, this test failed because of such an event.

testcases/t/130-close-empty-split.t

@@ -53,6 +53,11 @@ cmd 'kill';
 ($nodes, $focus) = get_ws_content($tmp);
 isnt($nodes->[0]->{id}, $split, 'split container closed');
 
+# clean up the remaining containers to ensure this workspace will be garbage
+# collected.
+cmd 'kill';
+cmd 'kill';
+
 ##############################################################
 # same thing but this time we are moving the cons away instead
 # of killing them