Fix the problems in the cookie implementation that hide test issues

[jdm]

web-platform-tests/wpt#8775 modified tests that were added in #11624. We should:

• figure out why they were passing in Servo while every other browser failed them
• add automated tests that specifically check those conditions, ensuring that other browsers pass them and Servo fails them
• modify our implementation to pass the new tests

[jdm]

Huh. We apparently don't run the cookies tests at all, and did not run them when when the test was added.

[jdm]

/cookies/path/match.html
------------------------
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: /
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: match.html
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: cookies
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: /cookies
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: /cookies/
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: /cookies/path/echo-cookie.html

These are the failures that exist on master.

[KiChjang]

With the patches applied on WPT upstream, the 2 remaining tests that fail are:

FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: match.html
FAIL `Set-Cookie` on /cookies/path/echo-cookie.html sets cookie with path: cookies

[pshaughn]

Are we still not running the cookies test directory?

[jdm]

Yeah, I forgot I filed this previous and filed #24869 last november.