Fix testing module dependency on httpx

[oliwenmandiamond]

When trying to update dodal to latest release of daq-config-server, have ran into a dependency issue when trying to import from testing module

026-07-28 13:37:10.446 [error] ImportError while loading conftest '/workspaces/dodal/tests/conftest.py'.
tests/conftest.py:13: in <module>
    from daq_config_server.testing import MockServerResponse, PathToMockDataDict
/cache/venv-for/scratch/bluesky_development/dodal/lib/python3.11/site-packages/daq_config_server/testing/__init__.py:1: in <module>
    from ._utils import make_test_response
/cache/venv-for/scratch/bluesky_development/dodal/lib/python3.11/site-packages/daq_config_server/testing/_utils.py:3: in <module>
    from httpx import Response
E   ModuleNotFoundError: No module named 'httpx'

This is because we have a make_test_response function which is in testing module which has a dependency on httpx and isn't a hard dependency for daq-config-server. Rather than adding the dependency, I have moved the function to tests only as that is where it is ever used. I have also checked plan repositories and it isn't used there either so safe to remove from testing and define it only where it is used.

[Relm-Arrowny]

LFTM, no reason for it to be inside the module.