brightness: Move slider test functions into util file

This CL moves three functions out of device_page_test into
device_page_test_util, since these functions will be used in an upcoming
CL. This CL also adds a new optional parameter "minimumValue" to
simulateSliderClicked to allow for cr-sliders with `min` values of
greater than 0.

Test: browser_tests -f="*DevicePage*"
Bug: b/328287584
Change-Id: I97deed01f2a3526185e8203d38bacb29b0eaaae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5377056
Commit-Queue: Camden Bickel <cambickel@google.com>
Reviewed-by: Wenyu Zhang <zhangwenyu@google.com>
Cr-Commit-Position: refs/heads/main@{#1277803}