add failing test case for allowParseErrors

[cheapsteak]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
  https://github.com/orgs/syntax-tree/discussions/114
• If applicable, I’ve added docs and tests

Description of changes

currently added a failing test for allowParseErrors

getting

     error: |-
        Expected values to be strictly equal:
        + actual - expected

        + '<a href="https://a?b&#x26;c"></a>'
        - '<a href="https://a?b&c">'

[ChristianMurphy]

from https://github.com/orgs/syntax-tree/discussions/114

it’s not a bug here. Rather, without it, this project would be completely broken.