Broaden pytest fixtures to more efficient scope

[diehlbw]

Problem Summary

Many of the tests' setup, particularly around data for loaders, is scoped more narrowly than necessary.

Impact

Running pytest suite creates more temp files than needed.

Possible Solution

Rather than have fixtures scoped to the individual test case, they could be reused - at the class or module level.
When files are created more for setting up test inputs, we shouldn't need to duplicate the files for each variant of inputs. While it's the safe side to error on, this will become impactfull as testsuite grows.