test: refactor test-util-inspect

* Handle a rejected Promise as that is expected to cause the process to
exit in a future version of Node.js. (Currently, it emits a warning.)

* Remove unneeded escaping in regexps

* Replace template strings with strings where there is no variable
substitution.

* A few minor readability changes (whitespace).

PR-URL: https://github.com/nodejs/node/pull/9804


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>