RUN keys in mockdata must be scalar but use last CONTROL key's dimensions

[philsmt]

I found a peculiar bug in extra_data.tests.mockdata.base while creating new virtual sources for EXtra tests. When the RUN keys are created in DeviceBase.write_control, the dims symbol from the last iteration of CONTROL keys is used as RUN keys cannot even define their own dimension and hence are assumed to be scalar.

EXtra-data/extra_data/tests/mockdata/base.py

Line 73 in 6a889bc

(1,) + dims, datatype, data=[value], maxshape=((None,) + dims))

We have not used non-scalar RUN keys before, but they can exist (abstractInterface e.g., something I was meaning to expose). So the proper fix is probably here to introduce a fourth element to the extra_run_keys tuple`?

@takluyver