Fix flaky tests that depend on verbosity level

[atugushev]

Partially addresses #1720.

log.verbosity is not thread safe. Better to pass verbosity as DI to avoid flaky tests.

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• Provided the tests for the changes.
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[atugushev]

Heh, test_sync_up_to_date is failed for the same reason. It seems to me that we should make piptools.logging.log thread safe or pass it as DI.

def test_sync_up_to_date(capsys, runner):
        """
        Everything up-to-date should be printed.
        """
        sync(set(), set())
        captured = capsys.readouterr()
>       assert captured.out.splitlines() == ["Everything up-to-date"]
E       AssertionError: assert [] == ['Everything up-to-date']
E         Right contains one more item: 'Everything up-to-date'
E         Full diff:
E         - ['Everything up-to-date']
E         + []

[atugushev]

FTR: to catch failed tests that depend on logs verbosity run the following command:

pytest tests/test_cli_sync.py::test_quiet_option tests/test_sync.py::test_sync_verbose -n0

[atugushev]

Superseded by #1761.