This change adds unit tests for the `aqt install` and `aqt install-qt`
commands, and establishes a pattern that can be extended for more
tests. This is intended to make it easier to increase test coverage of
parts of the codebase that are not yet covered by tests.
This uses the `pytest-socket` library to ensure that the tests do not
use any network IO. It also mocks `multiprocessing.get_context` to
prevent multiprocessing. This is necessary because multiprocessing
spawns child processes that have not been monkey-patched, which would
break the test.
These tests use py7zr to create mock 7z archives, which are installed
and patched in a temporary directory. The tests check the content
of the patched files and any output to stderr.
