unit tests should all use standardized fixtures

[alexkjames]

Currently our unit tests use a mixture of bespoke testing objects and well defined pytest fixtures. We should move to use pytest fixtures across the board (as much as is reasonable anyway)

• coherence
• correns
• ensembleseries
• geoseries
• multiplegeoseries
• multiplepsd
• multipleseries
• multivariatedecomp
• psd
• resolution
• scalogram
• series
• ssares
• tsbase
• tsutils
• [ ]

[CommonClimate]

Yes! Let me add that most of our tests use series that are on the long side. We should endeavor to use as few data points as possible to make the tests faster, thereby accelerating our unit test cycles.