Core: reduce scale factor for HadoopFileIOTest prefix tests

[jackye1995]

When running these tests, I notice it's taking a long time locally. I tried a few combinations

Current scale factors: 2 min 25 seconds scale factors (1, 10, 1000): 46 seconds scale factors (1, 5, 500): 24 seconds

Could we reduce the scale factor to (1, 5, 500)? It seems sufficient to me for the operation to run for half a minute for testing purpose. Is there anything that we need to test only in 2500 scale factor?

@edgarRd @danielcweeks