Add test.py script to run gtest and run-test

By default, it will run both run-test and gtest
If either of the two are explicilty asked for, it will
not run the other one.

Options:
--run-test : To run run-test
--gtest : To run gtest
-j : Number of jobs
--host: for host tests
--target: for target tests
All the other arguments will be passed to the run-test testrunner.

Test: ./art/test.py --host --target -j12 --run-test --gtest
Change-Id: I2fc4ebb900fb72db83165c6ae1615b8bdcfc33f4