add rule for too many test cases in test suite

This is based on the rule in https://code.google.com/p/robotframework/wiki/HowToWriteGoodTestCases#Test_suite_structure

"Should not have too many tests (max 10) in one suite unless they are data-driven."

Example output:

    W: 12, 0: Too many test cases (29) in test suite (TooManyTestCases)