conftest: Skip functional tests if splinter not importable
Although python3-selenium could be added to build deps, python3-splinter is not yet packaged.
Tests:
- Build package for unstable / bullseye-backports.
- Run autopkgtests.
- Run unit tests (as non-root and root user) in testing / stable container.
- Run essential functional tests.
Signed-off-by: James Valleroy jvalleroy@mailbox.org