Support escaped backticks in template literals

This is a totally valid use but since the lexer just looks for the next
backtick as a marker for the end of the literal, things fall apart
quickly.

I added this use to the fixture, as well as one for literals without templates... just in case.
(Also, I'm a bit surprised that we have octal literals now, and Number() understands them, but we don't have proper octal escape sequences! huh.)