Test harness CSS has an invalid selector

[AtkinsSJ]

The embedded stylesheet creates a rule with this invalid selector with two >s separated by a space:

table#results > thead > > tr > th {
	/* ... */
}

https://github.com/web-platform-tests/wpt/blob/253dee94a87a8a8467962cb53852fdd99d63a529/resources/testharness.js#L4962C1-L4966C3

It's also why there's no horizontal line separating the results table headings from the contents.

Currently:

With it corrected to table#results > thead > tr > th:

I'm making an issue instead of just fixing it because I'm not sure if people want that line. It's been missing for however long, so maybe it's better to remove that CSS rule entirely?

(My motivation: I always have Ladybird set up to log any CSS that fails to parse, to catch our bugs. It's easier to spot the real ones when there aren't false-positives from actually-invalid CSS on websites. 😉)