Disable pytest cacheprovider plugin in CI

Disable pytest cacheprovider plugin in CI

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 ("{}").

Given that we are not interested in this feature, disabling the
"cacheprovider" plugin globally and removing per-test removals of the
.cache directory seems like the best course of action.

(cherry picked from commit e1c30341076e0e1e76fbbb6cad6eb8086e6fc0e6)