refactor(stepper): clean up testing setup (#9772)

Currently the stepper tests are structured into completely different suites for vertical and horizontal steppers where the common tests are pulled out into functions which are called in each suite. This makes it harder to navigate and debug, in addition to flooding the logs with unnecessary data if one of the tests fails. These changes combine the tests into one suite with a couple of sub-suites that cover only the functionality that is exclusive to particular stepper variant.

Author: crisbeto