Update test_generation_strategy to utilize MBM models (#2734)

Summary:
Pull Request resolved: #2734

Models.GPEI points to the deprecated legacy Ax model. For most practical usage, it behaves identical to Models.BOTORCH_MODULAR (aka MBM), which is the recommmended model to use in Ax. This diff updates `test_generation_strategy` file to utilize MBM instead of legacy GPEI.

Reviewed By: Balandat

Differential Revision: D62164294

fbshipit-source-id: 4ea49e8ae038431600168661f747973cd1fcea40