Use mock instead of HTTPServer

[pared]

For functional tests we use tests.utils.httpd.StaticFileServer. To prevent mess associated with multiple tests trying to use server, we would prefer to use something that could mock responses, like https://github.com/bblimke/webmock (proposed by @shcheklein in #1540).
Task:
Remove StaticFileServer, use mocks instead.

[efiop]

@pared we could have both. The current one is suitable for functional tests, and mocked one would be suitable for unit tests. We will introduce a requirement for 100% coverage by unit tests in the future, so that is where mocked server could be used to quickly and efficiently test PRs, so then we could have a cron job or master tests to run functional tests.

Some how related: #907

[efiop]

Closing in favor of #907