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.
Migrated from D6511 (view on Phabricator)