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.
70 jobs for !4961 with mnowak/disable-pytest-cacheprovider-plugin in 14 seconds (queued for 8 seconds)
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  Unit
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  System
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