Add testcase which ensures that floating windows can be closed

See last commit

testcases/t/26-regress-close.t

@@ -0,0 +1,26 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Regression: closing of floating clients did crash i3 when closing the
+# container which contained this client.
+#
+use i3test tests => 1;
+use X11::XCB qw(:all);
+use v5.10;
+
+my $i3 = i3("/tmp/nestedcons");
+
+my $tmp = get_unused_workspace();
+$i3->command("workspace $tmp")->recv;
+
+$i3->command('open')->recv;
+$i3->command('mode toggle')->recv;
+$i3->command('kill')->recv;
+$i3->command('kill')->recv;
+
+
+my $tree = $i3->get_workspaces->recv;
+my @nodes = @{$tree->{nodes}};
+ok(@nodes > 0, 'i3 still lives');
+
+diag( "Testing i3, Perl $], $^X" );