provide WebServerListener for reusing

[dbu]

move WebServerListener to src/Test instead of tests/... and document it in testing-your-application.rst

[stof]

should it be provided in FOSHttpCache though ? It does not seem specific to our package at all. If the goal is to make it reusable, it may deserve being a separate package

[ddeboer]

Good point, see #223. 😉