spurious failure in test_cache_files

[ThomasWaldmann]

current master branch on travis-ci:

_________________ ArchiverCorruptionTestCase.test_cache_files __________________

[gw0] linux -- Python 3.8.6 /home/travis/build/borgbackup/borg/.tox/py38/bin/python

self = <borg.testsuite.archiver.ArchiverCorruptionTestCase testMethod=test_cache_files>
    def test_cache_files(self):
        self.cmd('create', self.repository_location + '::test', 'input')
        self.corrupt(os.path.join(self.cache_path, 'files'))
        out = self.cmd('create', self.repository_location + '::test1', 'input')
        # borg warns about the corrupt files cache, but then continues without files cache.
>       assert 'files cache is corrupted' in out
E       AssertionError: assert 'files cache is corrupted' in ''
.tox/py38/lib/python3.8/site-packages/borg/testsuite/archiver.py:3714: AssertionError

Bounty: https://www.bountysource.com/issues/93666717-spurious-failure-in-test_cache_files