Add a test which checks that we can find the stdin-wrapper script.

test/test_stdin_wrapper_path_can_be_found.vader

@@ -0,0 +1,4 @@
+Execute(We should be able to find the stdin-wrapper script):
+  silent! cd /testplugin/test
+
+  AssertEqual expand('%:p:h:h') . '/stdin-wrapper', ale#util#stdin_wrapper