use py.test for testing, refactor tests

[ThomasWaldmann]

pytest is way superior (and also more comfortable) compared to unittest.

less work to write tests, prettier / easier readable tests, more useful output (esp. on failure).

the existing tests can be taken "as is" as a starting point, pytest is able to run them.

later, more pytest specific features can be used to further improve.

be careful!

borg.selftest is special because it must not depend on pytest. it runs every time borg is invoked by a user.

see there: #6157 (comment)

These must not be converted to unittest:

from .testsuite.hashindex import HashIndexDataTestCase, HashIndexRefcountingTestCase, HashIndexTestCase
from .testsuite.crypto import CryptoTestCase
from .testsuite.chunker import ChunkerTestCase

SELFTEST_CASES = [
    HashIndexDataTestCase,
    HashIndexRefcountingTestCase,
    HashIndexTestCase,
    CryptoTestCase,
    ChunkerTestCase,
]

[ThomasWaldmann]

I made the necessary changes so py.test is used via tox (local testing) and travis ci.

I suggest that the existing tests are migrated to py.test slowly - if ones touches a specific area of the code anyway and writes new py.test-style tests, one can also migrate some of the existing tests.

Be careful, make sure tests work before and afterwards!

[hexagonrecursion]

You currently have a hierarchy of test classes with unittest.TestCase as the root. Several very nice features of pytest are unavailable in those tests. Once that stings especially badly is pytest.mark.parametrize.

https://docs.pytest.org/en/6.2.x/unittest.html?highlight=unittest#pytest-features-in-unittest-testcase-subclasses

I am going to start slowly purging the traces of unittest from your codebase.