Update html5lib-tests, install dependabot to open such PRs in the future

[untitaker]

~There's some tokenizer test failures, will try to fix them~

[untitaker]

I believe this is ready for review, but I have some concerns about how lol-html's test harness interacts with tokenizer tests. A test like this would be entirely valid to check into html5lib-tests:

{"tests": [

{"description": "Truncated open tag after script open tag",
"input": "<script><c",
"output":[["StartTag", "script", {}]],
"errors": [
  {"code": "eof-in-tag", "line": 1, "col": 11}
]}

]}

...and lol-html seems to fail it as it will additionally emit ["Character", "<c"]. That makes sense, after all lol-html doesn't want to modify the stream unnecessarily. But both parse5 and html5ever would pass this test.

[untitaker]

@jyn514 any chance this can be reviewed? I am also curious on y'all's opinion about the last testcase I posted