add basic e2e test for lastimport

[ghost]

Description

The lastimport plugin does not currently have any tests. This is a good starter for such a test suite. Mocking with RESPX is a WIP.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)
• Mock pylast with RESPX