Make a test that creates an NTNDArray using p4p

[coretl]

At the moment we use QSRV to do testing for PVA, but that can't do tagged unions. We should specifically test Signal[np.ndarray] using a p4p server with a StaticProvider, with a single SharedPV of type NTNDArray

Acceptance Criteria

• The np.ndarray test passes, and

  ophyd-async/src/ophyd_async/epics/core/_p4p.py

  Line 190 in 3d9f508

  ("epics:nt/NTNDArray:1.0", "v"): (np.ndarray, PvaNDArrayConverter),

  can be removed in favour of a line with U instead of v in it