Transform inline sample-folder into archive and adapt fixture accordingly

Since the setuptools-scm migration, the build fails because the sample-folder
within the repository held symlinks which setuptools-scm does not deal with
properly.

This adapts the conftest fixture to deal with the transformation from folder
to an archive (still holding the symlinks).

This fixes the build [1]

[1] https://jenkins.softwareheritage.org/job/DTSCN/job/tests/194/console

Related to T2105