Add test for Simulator.clear_probes

[hunse]

Motivation and context:

Simulator.clear_probes is not tested explicitly, so it's not required that backends implement it. This adds a test, which will prompt backends to implement this (though they can still explicitly opt out by skipping the test, of course).

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Non-code change (touches things like tests, documentation, build scripts)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.