Arca backend used in tests

[mikicz]

So I was looking at the failing test in test_arca_render and at first, I also had no clue what the problem is... But then after crawling through the logs and some experiments, I found the issue: the test is being run with arca.backend.CurrentEnvironmentBackend and the current environment has got naucse-render installed.

I really struggled to find why the hell CurrentEnvironmentBackend is used even though VenvBackend is configured in the tests (and even when I set the environ variable), but I finally found it in pytest.ini, the environ there overrides the VenvBackend in tests and even the backend provided in CLI.

I'm not exactly sure what backend you actually want to run it with so I'm not making a PR. Also, just as a heads up, VenvBackend does not work on Travis (https://github.com/travis-ci/travis-ci/issues/8589, issue closed because it was stale, but AFAIK still not resolved)