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.

Supersedes !4958 (closed).