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, here is the link:

http://www.ecma-international.org/ecma-262/6.0/#sec-get-regexp.prototype.flags

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)