test: improve test-assert

Add RegExp to check assert throws the expected AssertionErrors.

For the one with multiple flags is ok to hardcode the flags since the
spec indicates the ordering will always be `gim`:

Refs: http://www.ecma-international.org/ecma-262/6.0/#sec-get-regexp.prototype.flags
PR-URL: https://github.com/nodejs/node/pull/10916


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>