Use get_assigned_output for numbers

This prohibits the usage of workspaces assigned to other outputs in
create_workspace_on_output.

Eg, with config:
    workspace 1 output fake-0
    workspace 2 output fake-0
and 2 screens workspace 2 would be used for the second screen even
though it is assigned to the first one.

Also introduces a test for workspace assignments that includes the case
described above and some tests that don't fail in the next branch.

src/workspace.c

@@ -248,7 +248,9 @@ Con *create_workspace_on_output(Output *output, Con *content) {
         DLOG("Getting next unused workspace by number\n");
         int c = 0;
         while (exists) {
-            exists = (get_existing_workspace_by_num(++c) != NULL);
+            c++;
+            Con *assigned = get_assigned_output(NULL, c);
+            exists = (get_existing_workspace_by_num(c) || (assigned && assigned != output->con));
             DLOG("result for ws %d: exists = %d\n", c, exists);
         }
         ws->num = c;

testcases/t/297-assign-workspace-to-output.t

@@ -0,0 +1,60 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test assignments of workspaces to outputs.
+use i3test i3_autostart => 0;
+
+################################################################################
+# Test initial workspaces.
+################################################################################
+
+my $config = <<EOT;
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+fake-outputs 1024x768+0+0,1024x768+1024+0,1024x768+1024+768,1024x768+0+768
+
+bindsym Mod1+x workspace bindingname
+
+workspace 9 output doesnotexist
+workspace special output fake-0
+workspace 1 output doesnotexist
+workspace dontusethisname output doesnotexist
+workspace donotoverride output fake-0
+workspace 2 output fake-0
+workspace 3 output fake-0
+EOT
+
+my $pid = launch_with_config($config);
+
+sub check_output {
+    my ($workspace, $output, $msg) = @_;
+    is(get_output_for_workspace($workspace), $output, $msg);
+}
+
+check_output('9', '', 'Numbered workspace with a big number that is assigned to output that does not exist is not used');
+check_output('special', 'fake-0', 'Output gets special workspace because of assignment');
+check_output('bindingname', 'fake-1', 'Bindings give workspace names');
+check_output('1', 'fake-2', 'Numbered workspace that is assigned to output that does not exist is used');
+check_output('2', '', 'Numbered workspace assigned to output with existing workspace is not used');
+check_output('3', '', 'Numbered workspace assigned to output with existing workspace is not used');
+check_output('4', 'fake-3', 'First available number that is not assigned to existing output is used');
+check_output('dontusethisname', '', 'Named workspace that is assigned to output that does not exist is not used');
+check_output('donotoverride', '', 'Named workspace assigned to already occupied output is not used');
+
+
+
+exit_gracefully($pid);
+done_testing;