Revert "Began implementing a shared set of test models to speed up tests."

This reverts commit 22b7870e40a3ecf022b423de6cd867dcb35a6940.
Reverted the introduction of shared_models.

The recent improvements should make shared_models obsolete.

This reverts commit 1059da8de675442e84381d6366c0be254681753e, reversing
changes made to 4fa7f3cdd9bcf50ec4c7f64a31c1dfc02c375f46.