[cookies] Add test for default path calculation

[jugglinmike]

@mikewest This is the cause of many of the failures reported by wpt.fyi for cookies/http-state in Firefox, but it's difficult to understand the problem in those terms.

browser	status
Chrome 70	✔️
Edge 15	❌
Firefox 63	❌
IE 11	❌
Safari 12	✔️

[jugglinmike]

We should probably accommodate the aberrant behavior in the http-state tests since they are not intended to verify default path calculation. I've added a commit to do that.

Here are the results for that directory in Firefox on master today (i.e. ./wpt run --no-restart-on-unexpected firefox cookies/http-state)

web-platform-test
~~~~~~~~~~~~~~~~~
Ran 232 checks (11 tests, 221 subtests)
Expected results: 19
Unexpected results: 213
  subtest: 213 (213 fail)

Restarting after failures alleviates the problem to some degree, though not completely and at the cost of a slower time-to-results (./wpt run firefox cookies/http-state)

web-platform-test
~~~~~~~~~~~~~~~~~
Ran 232 checks (11 tests, 221 subtests)
Expected results: 77
Unexpected results: 155
  subtest: 155 (155 fail)

Firefox fares much better with this patch applied (./wpt run firefox cookies/http-state)

web-platform-test
~~~~~~~~~~~~~~~~~
Ran 232 checks (11 tests, 221 subtests)
Expected results: 169
Unexpected results: 63
  subtest: 63 (63 fail)

...and those improvements persist even when running without restarting after failures (./wpt run --no-restart-on-unexpected firefox cookies/http-state)

web-platform-test
~~~~~~~~~~~~~~~~~
Ran 232 checks (11 tests, 221 subtests)
Expected results: 169
Unexpected results: 63
  subtest: 63 (63 fail)

[mikewest]

LGTM, thank you for digging into this!