Add fake PathProvider to startup module

[callumforrester]

This is a workaround (not a fix) for #784, so won't close it. The replacement of PathProvider in dodal will eventually provide the proper fix. For now I think we should just pretend to write to a directory with our start-up file, despite the fact that none of the simulated detectors actually write files.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.46%. Comparing base (b605942) to head (3b3a228).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #783   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files          38       38           
  Lines        2111     2111           
=======================================
  Hits         1973     1973           
  Misses        138      138           

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