Make the sign placement test pass more often by using another shell

test/sign/test_linting_sets_signs.vader

@@ -34,7 +34,7 @@ Before:
   \ 'name': 'testlinter',
   \ 'callback': 'TestCallback',
   \ 'executable': has('win32') ? 'cmd' : 'echo',
-  \ 'command': 'echo foo bar',
+  \ 'command': has('win32') ? 'echo foo bar' : '/bin/sh -c ''echo foo bar''',
   \})