Expands lexer tests for astring with closer attention to ABNF in spec

@alienscience As you're writing the lexer I've left this as a pull request for your review instead of just forcing it on you.

Some of the test cases do not pass, although I've left them commented out.  These need your eye as I'm not confident I can say they should or shouldn't pass - only that the RFC seems to allow them.

I've also reproduced the entire <astring> rule from the spec in the comment as a sort of tree - perhaps unnecessarily but just out of temporary convenience.  I'm conscious that this will come out as one long weird string under godocs in the long-term so I can happily remove it.

Still lacks a complete set of test cases (e.g. <CTL> is not tested)