Add Activity Testing API and check command to setup.py

./setup.py check now invokes tests in tests/ directory. "tests" directory should
have integration(UI tests) and unit(unit tests) sub directories, but it isn't
necessary to have any dir. of the above.
/setup.py has optional args to invoke only the unit tests or ui tests.
The commands are ./setup.py check unit and ./setup.py check integration respectively.

Example tests added to calculate - https://git.sugarlabs.org/~svineet33/calculate/svineet33s-calculate