Remove test_utils from default arrow-rs features

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

This brings in a non-trivial amount of addition code and logic that is not relevant to downstream crates

Describe the solution you'd like

We should remove it from the default features set, as a first step to potentially splitting it out into its own crate (which we don't publish).

Describe alternatives you've considered

Additional context