Ignore test on Windows

The userscript is a bash script and there is no bash on windows.
One solution could be to use a python userscript, but there may be
other issues (file associations), too.

tests/integration/features/spawn.feature

@@ -16,6 +16,7 @@ Feature: :spawn
         When I run :spawn echo {url}
         Then "Executing echo with args ['about:blank'], userscript=False" should be logged
 
+    @posix
     Scenario: Running :spawn with userscript
         When I execute the userscript open_current_url
         And I wait until about:blank is loaded