change analysis test to use the simulated_spectra fixture

[eteq]

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

[nmearl]

@eteq the analysis functions use the simulated_spectra fixture where appropriate (and otherwise construct necessary models/spectra to test specific analysis tasks like equivalent width on a known Gaussian). Are there other areas you had in mind that haven't made the transition?