[MNT] - Update tests & small plot bug fixes

[TomDonoghue]

This started as a quick bug fix for a plot thing, and then escalated a little....

Updates in this PR:

• Bug fix: fixes labelling on a phase instantaneous frequency so it plays more nicely with subplots
• Makes test plots more realistic, so that saved out plots are more representative and easier to check
• Simplifies test settings and fixtures**

**We were starting to get a slightly unwieldy collection of settings and fixtures. As I was trying to generalize the plots, using more realistic signals, it became clear the mapping between test signals and settings was not super clear. So, a quick clean up which basically simplifies test signals to have one regime of (N_SECONDS, FS) for fixtures defined in conftest, and then makes any tests that need idiosyncratic custom test object definitions create their own local simulations.

Note: the changes here should be relatively simple, even if the diff is somewhat large. Should only need a relatively quick check.

[ryanhammonds]

Everything looks good here!

[TomDonoghue]

Thanks for the check!