JP-1954: Changed tests to parametrize the choice of ramp fitting algorithm

[kmacdonald-stsci]

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The tests set up a class to parametrize testing ramp_fitting using ordinary least squares (OLS) and generalized least squares (GLS) as algorithms to compute slopes and variances for ramps. However, the tests had OLS hardcoded as the algorithm to use, so the same test twice, instead of testing GLS.

Addresses #5826 Fixes JP-1954