P4P tests are a bit slow

```
========================================================================================================================== slowest 10 durations ===========================================================================================================================
3.01s call tests/transport/epics/pva/test_p4p.py::test_command_method_put_twice
2.88s call tests/transport/epics/pva/test_p4p.py::test_scan_method
1.26s setup tests/test_docs_snippets.py::test_snippet[docs/snippets/static01.py]
0.73s setup tests/transport/epics/pva/test_p4p.py::test_ioc
0.73s setup tests/transport/epics/ca/test_softioc_system.py::test_ioc
0.62s call tests/transport/epics/pva/test_p4p.py::test_command_method
0.61s call tests/transport/epics/pva/test_p4p.py::test_more_exotic_dataypes
0.50s call tests/transport/epics/pva/test_p4p.py::test_block_flag_waits_for_callback_completion
0.43s call tests/transport/epics/ca/test_softioc_system.py::test_ioc
0.21s call tests/transport/epics/pva/test_p4p.py::test_pvi_grouping
```

Investigate and try to get each test down to ~1 second

Running fastcs as an async task like [this](https://github.com/DiamondLightSource/FastCS/blob/aad1a85fee0039ef2692d493d28ccaacfd382da6/tests/transports/epics/pva/test_p4p.py#L473) instead of a subprocess may help.