Execute tests on CI system

See merge request mpsd-cs/mpsd-software-environments!4

allow CI to fail if ruff or black is unhappy, and format source to make it pass

See merge request mpsd-cs/mpsd-software-environments!3

- no attention or change to contents
- tests not executed

triggered by discussion https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/merge_requests/3#note_803951

- didn't pay attention to content
- didn't intend to change content or behaviour
- didn't execute tests

Resolve "First draft for user interface for top level install command"

Closes #2 and #1

See merge request mpsd-cs/mpsd-software-environments!1

first attempt to have tests (fake tests only)

See merge request mpsd-cs/mpsd-software-environments!2

(will do style changes in separate MR / commit)

(We need some CI to catch at least the trivial mistakes like the one from the
last commit.)

as the function supports one or more arguments.

See discussion at
https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/issues/1#note_799664

(fix https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/issues/2)