Improve `ModeOfOperation.Test` usage

This mode was [added as a convenience](https://github.com/cnpem/sophys-cli-core/commit/fee461197c0d12c4dd7388c9798c96540445a0e7) for running tests on plan magics. However, it does not allow us to inspect the final objects being sent to the RunEngine (local mode) or to the httpserver (remote mode). Ideally we should perhaps have access to those objects in the tests too. Besides, the extra boilerplate to create that test output is annoying for creating new tests.

A big obstacle for doing so is figuring out how to properly clean up the magics between each test, as they are registered as [class attributes in a common module](https://github.com/cnpem/sophys-cli-core/blob/678f686aea1c41f686c34792c7c344f5f10550fe/src/sophys/cli/core/magics/plan_magics.py#L426).