change analysis test to use the simulated_spectra fixture

sort of a "housekeeping" item, but it would be nice to try to get the analysis tests as consistent as some of the others in using fixtures consistently. That is, the analysis tests currently make their own test datasets instead of using shared spectra from the spectral_examples.py module

Locate the analysis tests and the shared spectral_examples.py module, then inspect the simulated_spectra fixture and how the tests currently construct datasets. Update the analysis tests to use that shared fixture consistently, and run the relevant analysis test suite to confirm it still passes.

Written by the indexing model from the issue text.