test: remove unneeded escaping in template strings

The no-useless-escape rule in ESLint did not previously flag certain
unnecessary escaping in template strings. These will be flagged in
ESLint 3.8.0.

PR-URL: https://github.com/nodejs/node/pull/9112


Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>