simpleperf: merge test_monitor.py into do_test.py.

1. Use multiprocessing in do_test.py to run tests in child processes.
2. Add -d option to run test on multiple devices in parallel.
3. Add -r option to repeat testing.
4. Remove test_monitor.py.

Bug: 182507493
Test: run test.py.
Change-Id: I1ab2874d8a44bc9ad2bf1830864ddb8e467fbd8b
2 files changed