simpleperf: split python tests.

Currently all python tests are in test.py, making it too big to
maintain. So this CL does below changes:
1. create test directory, and move test.py, test_monitor.py and
script_testdata to the directory.
2. move test.py to do_test.py, and split its test cases into different files.
3. create a new test.py as an entrypoint for testing.
4. use python3 in testing.

Bug: 182507493
Test: run test.py.
Change-Id: Id880036c5115d00da74ef1b1b948cb8f63b928f9
35 files changed