Move Transition test utils to tests source folder

This CL moves the test utils in Transition.kt from
scene/tests/utils/src/ to scene/tests/src/. Unfortunately utils in
scene/test/utils/src are compiled separately from the STL lib and test
sources, which means that they can't access internal members (because of
b/240432457). I will need this file to access internal members in
ag/26597678, hence the move.

Bug: 290930950
Test: pure move + presubmits
Flag: N/A
Change-Id: I5270eaf8ba373354ff59cbb039a47c3916e36888