Bug 991653 - [v1.3] Backport test_power_button_long_press.py test to v1.3

This pulls in a commit for:
Bug 968751 - Write a test to open long-press power button menu
Merge pull request #17991 from bobsilverberg/power_button

Bug 991653 - [v1.3] Backport test_power_button_long_press.py test to v1.3