Replace zhinst-dependent tests with a mock backend
After !887 (merged) gets merged the zhinst
backend will be optional. The tests that depend on the zhinst
backend are now a bit hackily ignored:
- if the CI job has python 3.8 or 3.9 installed it will run zhinst-related tests.
- if the CI job has python 3.10+ installed, these tests are skipped.
These tests should ideally be replaced by the (to-be-developed) mock backend.