TST: Revert hardcoding in test_bls.py and test_models.py #11567

See #10644 (review) from @astrofrog

• astropy/timeseries/periodograms/bls/tests/test_bls.py
• astropy/modeling/tests/test_models.py

I think in a follow up we should really try and get rid of the hard coded arrays. I have some ideas for finding a seed so if someone opens the issue it can be assigned to me.