Speed up tests

[DominicOram]

We have a few tests that are taking >1s. We should speed them up:

============================================================================== slowest durations ===============================================================================
3.42s call     tests/test_cli.py::test_cli_version
3.01s call     tests/devices/unit_tests/test_focusing_mirror.py::test_mirror_set_voltage_sets_and_waits_settle_timeout_expires
1.02s call     tests/devices/unit_tests/test_apple2_undulator.py::test_phase_time_out_error
1.01s call     tests/devices/unit_tests/test_apple2_undulator.py::test_gap_time_out_error
1.01s call     tests/devices/unit_tests/test_apple2_undulator.py::test_jaw_phase_time_out_error

It's likely that the unit_tests ones are poor mocking but the test_cli_version maybe makes more sense moved to the system tests.

Acceptance Criteria

• All unit tests take <1s