fix failing tests that exepect an envvar

[noslouch]

if MAPBOX_API_KEY is not set in the environment, test_pass_kwargs fails.

noticed this when trying to run tests locally. the D prefix defers to values already set in the environment.

you may not want to introduce a new dependency to your project (namely pytest-env), and I would understand that, but just thought I'd bring this to your attention with a suggested fix. take it or leave it! this is a very useful library. thanks!

[eyeseast]

It works if I add the --api-key option, so I'd rather do that than add a dependency. Thanks for catching this, though. I'll fix this on another branch.