Update cross validation tests to reduce mocking (#3745)

Summary:
Pull Request resolved: #3745

They now use data from an actual experiment and an actual `Adapter` rather than a mock object with all mock methods.

Reviewed By: Balandat

Differential Revision: D74342457

fbshipit-source-id: 2795bbe74318de5cfc37935f2f5e7b622212003e

Author: saitcakmak

ax/core/trial.py

@@ -9,9 +9,7 @@
 from __future__ import annotations
 
 from functools import partial
-
 from logging import Logger
-
 from typing import Any, TYPE_CHECKING
 
 from ax.core.arm import Arm

ax/modelbridge/cross_validation.py

@@ -123,7 +123,7 @@ def cross_validate(
     arm_names_rnd = np.array(list(arm_names))
     # Not necessary to shuffle when using LOO, avoids differences in floating point
     # computations making equality tests brittle.
-    if folds != -1:
+    if folds != n:
         np.random.shuffle(arm_names_rnd)
     result = []
     for train_names, test_names in _gen_train_test_split(