Move development reqirements to dev-requirements.txt

In some cases, such as when working inside a virtual environment, it can
be desirable to install all dependencies for running tests using `pip`.
This is currently not possible since setuptools does not support
installing these dependencies in a virtual environment (by design).
(See https://stackoverflow.com/a/21003259.)

To circumvent this issue, move such requirements to
`dev-requirements.txt` file that can be used easily with
`pip install -r`. This also enables tests to be run directly using
`pytest`, which can be more convenient than `-addopts` approach when one
needs to add multiple options.

This will also be useful in creating better testuite images, and fix
some of the issues noticed in
buildstream-docker-images!56.