Add a lint rule for /> syntax on non-void HTML elements

[zcorpan]

See #40473

[zcorpan]

How do I run the lint tests locally?

[foolip]

@zcorpan running tox in tools/ will probably run those tests. Then you probably have to read the source to figure out how to narrow it down to just run the tests you want.

[zcorpan]

I needed to run flake8 -v --append-config=/Users/simonpieters/git/web-platform-tests/wpt/tools/flake8.ini (with -v) to see what caused the failure.

./lint/rules.py:508:142: E501 line too long (542 > 141 characters)

Now fixed. Also removed wbr from the regexp since it's a void element.

[foolip]

Changes look good. Should be possible to write a test for this by copying other lint rule tests, but I'll leave it to you to judge if that's worthwhile or not.

[zcorpan]

Done, and confirmed that it works locally with:

pytest --cov=tools --cov-report=term

Changing the test to expect no errors makes it fail.