Add two more tests that currently fail.

Each fails for a different reason.  The first fails because it matches
where Perl does not.  (It's not clear whose bug this is.)  The second
fails because we're not waiting until the regex has finished compiling
before validating register names.
Add two more tests that fail.

These are like the previous two tests added in the previous commit,
except they deal with "self-referential" backreferences rather than
forward backreferences.
Document subpattern references in the html documentation.
Add subpattern reference commentary to docs on *ALLOW-NAMED-REGISTERS*

Also, add some FIXME comments to return to later.