tests: Fix execution when building debian packages

Recent changes in swh-web tests broke the debian package builds.

When building debian packages, the pytest command is not available in build
environment and must be executed through "python3 -m pytest" instead.

Debian buster packages a quite old version of pytest (3.10.1) and some
pytest hooks parameters and CLI options are missing compared to upstream
version so workaround these issues.

Also silent non critical warnings related to the use of hypothesis and
pytest function scoped fixtures as they add a lot of noise in pytest
reports.