Update tests to skip tests if dependencies are missing

[djhoese]

Would it be a bad idea to make tests that don't have the necessary dependencies mark themselves as skipped? For example, the mitiff writer's (@TAlonglong) tests require the libtiff (pylibtiff) python library which isn't always the easiest to install. To have the tests fail may be too strong of a response, but I could see this being bad on CIs where we would want to be alerted in the most obvious way (failed tests) if dependencies are missing.

Thoughts?

[djhoese]

No comments since making this. I'm closing this as we've generally decided that all tests should be runnable on CI and someone should expect failures for missing dependencies. Now that we have pytest it is much easier to do pytest satpy/tests/test_some_module.py.