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.
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