Move (predictive) back tests into PredictiveBackHandlerTest

This CL simply moves the tests for (predictive) back into their own
file. This is a pure clean up that shouldn't impact anything.

Bug: 350705972
Test: atest PredictiveBackHandlerTest
Flag: EXEMPT test only
Change-Id: I6020701c4f4b4c20e70a9de6a36aaa15097bd747