Address flaky unit tests

[drewoldag]

Some of the unit tests seem to be non-deterministic. See for example, the smoke test runs here: https://github.com/lee-group-cmu/FlexCode/actions/workflows/smoke-test.yml

Nothing has changed, but the unit tests are not reliably passing and thus send false alerts.

[drewoldag]

For reference, the test that failed: FAILED tests/flexcode/test_cv_optim.py::test_coef_predict_same_as_predict_custom_model