Add reboot_test

This test spawns several services backed by /system/bin/yes executable,
and then stops them either while SIGTERM or SIGKILL.

Ideally we want to unit test more of reboot logic, but that requires a
bigger refactoring.

Test: atest CtsInitTestCases
Bug: 170315126
Bug: 174335499
Merged-In: Ife48b1636c6ca2d0aac73f4eb6f4737343a88e7a
Change-Id: Ife48b1636c6ca2d0aac73f4eb6f4737343a88e7a