Address slow tests

[abbiemery]

A few of the unit tests are taking quite a while to complete (~10 seconds each) so I've made a list of them.

tests/unit_tests/core/test_context.py::test_add_devices_from_module
tests/unit_tests/core/test_context.py::test_add_devices_and_plans_from_modules_with_config 
tests/unit_tests/test_example_code.py::test_example_device_module_is_detectable 

Acceptance Criteria

• Make them take < 3 seconds.

[callumforrester]

Could adopt a time limit for tests, same as ophyd-async