tests: allow passing the INTEGRATION_TEST variable from the commandline

this allows to run

INTEGRATION_TEST=1 tox -e <env> test/test_<module>.py

which is very useful when testing locally. Otherwise, tox will clean the
environment, such that INTEGRATION_TEST will not be passed to the test
runner.

tox.ini

@@ -17,6 +17,7 @@ files = beets beetsplug beet test setup.py docs
 deps =
     {test,cov}: {[_test]deps}
     lint: {[_lint]deps}
+passenv = INTEGRATION_TEST
 commands =
     test: python -bb -m pytest -rs {posargs}
     cov: coverage run -m pytest -rs {posargs}