Some pva tests hang forever if the subprocess FastCS fails to start

[GDYendell]

Describe the bug, including a clear and concise description of the expected behaviour, the actual behavior and the context in which you encountered it (ideally include details of your environment).

Steps To Reproduce

Steps to reproduce the behavior:

1. Break example_p4p_ioc so that it fails to run the IOC
2. Run test_p4p.py::test_ioc

Acceptance Criteria

• All tests timeout and produce a test failure instead of hanging

[GDYendell]

ophyd-async has a global timeout for all test that we could copy