Test for invalid "$anchor" syntax

[handrews]

• No # allowed (e.g. "$anchor": "foo" is valid, `"$anchor": "#foo" is not)
• No JSON Pointer syntax (e.g. `"$anchor": "/a/b" is invalid)

There are many other possible invalid forms, but these are the two that people are most likely to think might work.

[Julian]

This one depends on #244 -- which we should totally do but ugh yeah has not been something I've managed to hit so far... :(

[ssilverman]

How does this depend on #244? I'm not doubting, just not sure how.

[Julian]

The above is wrong (my comment), we indeed can do this.