add some negative tests

[rogpeppe]

The tests currently have some examples that should validate, but does
not include any that should not validate. I checked by replacing
schema.json with the following:

{
  "$schema": "http://json-schema.org/draft-07/schema#"
}

and all the tests still passed, which does not seem ideal.

It would be good to have some negative tests too.

[angusfretwell]

Hi @rogpeppe 👋

Thanks for raising the issue (and for your other recent contributions).

Agreed, the tests leave something to be desired.

I'll add this to our internal backlog, but we'd be happy to accept a PR if you like 😄