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).