Use pip-sync to keep test environments up-to-date

According to https://tox.readthedocs.io/en/latest/example/general.html#dependency-changes-and-tracking, requirements changes are not well/completely tracked:

(...) requirement file content changes are not tracked.

This leads to outdated test envrironments => potential bugs uncatched (NXDRIVE-2169 ...) => very bad for the product.

We could use pip-sync to force updating requirements before doing anything.