test: refactor test-crypto-timing-safe-equal

Add RegExp arguments to throws assertions.

PR-URL: https://github.com/nodejs/node/pull/9843


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>