Pipeline tasks can return multiple outputs to unpack

[jrs65]

• test(pipeline): ensure multiple output pipeline tasks are supported
• fix(pipeline): pipeline tasks can return multiple outputs to unpack

[rikvl]

Hope I understood what's going on here. If so, I think this only requires one or two small changes.

[ljgray]

The key functionality here was added in a different PR, so this is now just adding tests

[ketiltrout]

This seems fine.

Though, importing conftest directly, instead of just setting up a bunch of fixtures is a weird way of doing things. But I don't think this PR is the place to address that.