Added a system() method to the TestBase class of lldbtest.py, which is actually

taken from Python 2.7's subprocess.check_output() convenience function.  The
purpose of this method is to run the os command with arguments and return its
output as a byte string.

Modified hello_world/TestHelloWorld.py to have two test cases:

o test_with_dsym_and_run_command
o test_with_dwarf_and_process_launch_api

with the dsym case conditioned on sys.platform.startswith("darwin") being true.
The two cases utilize the system() method to invoke "make clean; make MAKE_DYSM=YES/NO"
to prepare for the appropriate debugging format before running the test logic.

llvm-svn: 112530