Tests: organized and revived project/tp functional tests

Note that the translation project tests have been simply converted to the
pytest style without further changes so have been marked with
`pytest.mark.xfail`. Since these test features we don't have/use, these
are only kept for upstream.
Tests: monkeypatch `cached_property`

The cached values were causing trouble when DB objects were involved since
they referenced to "old" models and then every single test was flooded by
`IntegrityError`s.

Not the nicest way to replace a decorator but we need to do that before
the modules using it are imported.
Tests: improved docs and added basic guidelines
Ignore included requirements files when parsing requirements file
Tests: be explicit about the tests directory

Although the natural environment for tests are UNIX-like systems, this
improves cross-platform compatibility so tests can also be run on Windows
machines. The change simply avoids the forward slash.
Tests: Run tests on Travis
Tests: Avoid creation of a virtualenv specific to tests