[cookies] Correct utility function and tests

[jugglinmike]

The cookie-helper.sub.js utility script includes
set_prefixed_cookie_via_http_test, a function that defines sub-tests
using the promise_test API. Previously, it included the following
code:

promise_test(t => {
  var postDelete = _ => {
    // (elided)
  };

  if (!options.origin) {
    return postDelete;
  } else {
    // (elided)
  }
});

The promise_test function does not recognize return values which are
functions, so returning the postDelete method had no effect, and as a
result, the generated tests performed zero assertions. Because none of
the consumers of set_prefixed_cookie_via_http_test specified a value
for the origin option, all invocations were effected by this bug.

Correcting the problem surfaced a number of errors in the tests. In the
interest of atomicity, this patch attempts to address them all:

• The logic intended to defensively remove cookies prior to testing was
  implemented using document.cookie. Because some tests create cookies
  which include the HttpOnly attribute, the DOM API cannot remove
  cookies in all cases. This patch refactors the solution to remove
  such cookies via an HTTP request. It also assumes the environment is
  initially clean and instead expresses the concern via an asynchronous
  "cleanup" function. (This change necessitated an extension to the
  set.py script so that it could be used to expire cookies.)
• The test name __secure.header.html incorrectly asserted that a
  cookie set with the Secure attribute could be observed in a
  non-secure context. This patch corrects the expectation.
• The test named __secure.header.https.html incorrectly asserted that
  a cookie set with a foreign Origin attribute could be observed from
  the current origin. This patch corrects the expectation.

---

This patch does not affect the test results in Firefox or Chrome. The commands:

$ xvfb-run --auto-servernum ./wpt run firefox cookies/prefix
$ xvfb-run --auto-servernum ./wpt run chrome cookies/prefix

...both continue to report the following:

web-platform-test
~~~~~~~~~~~~~~~~~
Ran 95 checks (9 tests, 86 subtests)
Expected results: 95

[mikewest]

Thank you, this patch looks correct to me. I very much appreciate you digging in.

LGTM.