CLI metadata populate integration tests

[sbesson]

While working on #5, the absence of integration test for CLI metadata populate prevents the detection of breakages in the way the ParsingContext is called via the CLI plugin. Given the work on refactoring the API, adding such tests is probably necessary in the context of 0.3.0.

This could involved some migration of the fixtures classes in https://github.com/sbesson/omero-metadata/blob/v0.2.2/test/integration/metadata/test_populate.py if covering the various population use cases is valuable. If that becomes too complex, testing the CLI wrapping logic might be a good first step.