Fixes #954
Enables us to run unit tests with pytest -m "not external_tests" rather than pytest -m "not (s03 or adsim)"

Instructions to reviewer on how to test:

1. Confirm that running pytest "not external_tests" correctly skips s03 and adsim tests but nothing else

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}