Add basic tests for WebEnginePartCookieJar

Summary:
Currently, this only tests that cookies are correctly added to
KCookieServer in response to the QWebEngineCookieStore::cookieAdded
signal.

There are some limitations:
- ideally, to test the correct behaviour of WebEnginePartCookieJar::cookieAdded,
  one should call QWebEngineCookieStore::setCookie, so that
  QWebEngineCookieStore processes the cookie and emits the cookieAdded
  signal as it would in the real application. Unfortunately, in the
  tests, calling setCookie doesn't trigger the cookieAdded signal.
  Because of this, I manually emitted the signal. However, this bypasses
  any processing by the cookie store (such as calling
  QNetworkCookie::normalize, which I do manually);
- I don't know how to start a separate instance of the Cookie Server
  service, so the tests use the default one. This has two drawbacks:
  it's impossible to start with a clean state and the tests pollute the
  user's configuration (this second issue could be avoided by adding
  clean up code to each test)

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D15381