Disable pytest cacheprovider plugin on Ubuntu Xenial
The pytest "cacheprovider" plugin produces a .cache/v/cache/lastfailed
file, which holds a Python dictionary structure with failed tests.
However, on Ubuntu 16.04 (Xenial), the file is created even though the
test passed and the file contains just an empty dictionary ({}).
Disabling the "cacheprovider" plugin on Xenial and removing per-test
removals of the .cache directory seems like the best course of action.
This is a v9.16-only change because Xenial was removed from main and we don't employ pytest on v9.11.
This MR supersedes !4949 (closed).