Some tests fail if imageio-ffmpeg is not installed

[olebole]

imageio-ffmpeg is not packaged for Debian yet, so our tests are run without. There are two tests that fail if the package is not installed:

• test_preferring_arbitrary, and
• test_missing_format

While I can patch the test file to skip these locally, it may be useful to change them so that the tests run independently of the imageio-ffmpeg package.