Tests: Disable the ":lang respects escaped backslashes" test

Tests: Disable the ":lang respects escaped backslashes" test

Firefox 114+ no longer match on backslashes in `:lang()`, even when escaped.
It is an intentional change as `:lang()` parameters are supposed to be valid
BCP 47 strings. Therefore, we won't attempt to patch it.
We'll keep this test here until other browsers match the behavior.

Fixes gh-5271
Closes gh-5277
Ref https://bugzilla.mozilla.org/show_bug.cgi?id=1839747#c1
Ref https://github.com/w3c/csswg-drafts/issues/8720#issuecomment-1509242961

(cherry picked from commit 62b9a2583460c2384f6de1d0a6aeaf05e51d523b)