Add test to check that cookies are imported from KCookieServer at creation

Summary:
In theory it should be possible to test this by connecting to
QWebEngineCookieStore::cookieAdded signal. However, in tests this signal
is not emitted (or, at least, it's emitted after the test has run). To
work around this issue, if the BUILD_TESTING cmake variable is true, a new
instance variable is created for WebEnginePartCookieJar and cookies added on
creation are stored there. The contents of this variable are then checked by
tests.

Currently, the tests fail because there's a small difference (of the
order of milliseconds, it seems) on the expiration dates. These tests
are currently marked as expected failures

Test Plan: run autotests

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D16407