Inject mock zebra arm logic into device

[DominicOram]

Following bluesky/ophyd-async#1129 we should inject logic into the device wherever we're using it a lot in mocks.

For the zebra we add the following logic downstream:

    def mock_side(demand: ArmDemand):
        set_mock_value(zebra.pc.arm.armed, demand.value)
        return NullStatus()

    zebra.pc.arm.set = MagicMock(side_effect=mock_side)

we should instead inject it.

Acceptance Criteria

• Above code is injected into the device mock
• Downstream code no longer does the inject and tests still work