Modularise pytest fixtures

[willGraham01]

Closes #353 |

Breaks out the fixtures and command-line options in conftest.py into the (new) tests/fixtures directory.

• conftest.py now discovers all pytest-related options (EG CLI flags) and objects (EG fixtures) defined inside *.py files in tests/fixtures. Sub-directories are also recursed into.
• The command-line addoptions have been moved to pytest_cli_options.py.
• The signal generation fixtures also have their own file.
• The fixtures used to cache and load cached data have their own fixtures/caching subdirectory & are further broken down into files. Some "module-level" constants have been made into fixtures to allow their discoverability elsewhere.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (1e1fc89) to head (3585560).

❗ There is a different number of reports uploaded between BASE (1e1fc89) and HEAD (3585560). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (1e1fc89)	HEAD (3585560)
	10	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #373       +/-   ##
===========================================
- Coverage   96.09%   79.90%   -16.20%     
===========================================
  Files          32       32               
  Lines        3458     3458               
===========================================
- Hits         3323     2763      -560     
- Misses        135      695      +560     

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