Fixes #17297.  Changed relative mouse position to absolute, and added automated RangeSlider tests.

(cherry picked from commit f21df6fbd6063ca4295d8932d12ffa463a36a365)

Conflicts:
	form/_RangeSliderMixin.js
	form/tests/test_RangeSlider.html

(cherry picked from commit 9ec390c901f7b875ddc34d5efbc06f9223033ad6)