test-console: streamline arrow fn and refine regex

removed unneccessary curly braces and return statement from
inspect arrow function
updated `assert.throws` regex to look for exact match at start
of string

PR-URL: https://github.com/nodejs/node/pull/11039


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>