The specification contains an explicit informative NOTE explaining that
the 'm' flag does not effect the behavior of the '^' assertion. Add a
test to verify this.
ES2015 reads:
> RegExpUnicodeEscapeSequence :: u{ HexDigits }
>
> - It is a Syntax Error if the MV of HexDigits > 1114111.
Use the MV 0x110000 to assert the lower boundary of values which are
expected to produce the SyntaxError.