Simpleperf: add separate cpu_hotplug_test.

cpu_hotplug_test runs much longer than unit tests, so separate it.
It uses the same strategy as the shell scripts I used to find all
kernel bugs.

Bug: 26032318
Bug: 25193162
Change-Id: I2f26d4f0d65e680b597e40cce9e775b9568599ef
3 files changed