test(material/slider): clean up harness testing setup

[crisbeto]

Now that we know that the MDC-based slider won't have the same API as the non-MDC version, we can avoid sharing the test harness tests and clean up some of the logic.

[devversion]

LGTM. Tests fail because the mdc-slider/testing package references the harness_tests_lib target. That should be removed to make CI green.

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