Port the python api decorator to use test categories

Summary:
Per discussions on the mailing list, I have implemented a decorator which annotates individual
test methods with categories. I have used this framework to replace the '-a' and '+a'
command-line switches (now '-G pyapi' and '--skip-category pyapi') and the @python_api_test
decorator (now @add_test_categories('pyapi')). The test suite now gives an error message
suggesting the new options if the user specifies the deprecated +/-a switches. If the general
direction is good, I will follow this up with other switches.

Reviewers: tberghammer, tfiala, granata.enrico, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14020

llvm-svn: 251277