Add a (disabled) code-smell test for detecting _ variables

We are reserving the _ identifier for i18n work.  Code should use the
identifier dummy for dummy variables instead.

This test is currently skipped as someone needs to generate the list of
files which are currently out of compliance before this can be turned
on.

test/sanity/code-smell/skip.txt

@@ -1 +1,2 @@
 inappropriately-private.sh
+no-underscore-variable.sh