mir: Uncommented and fix compilation issues for rewrite tests.

During the MIR refactor, there were some tests that got commented out
during the work, but were never uncommented afterwards. Oops!
This fixes that.

Refs: ENG-1621
Change-Id: Icae7c9b964644073e84dd9427008796c718a192b
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/3906
Tested-by: Buildkite CI
Reviewed-by: Griffin Smith <griffin@readyset.io>