Add an initial version of test that exercise the lldb commands: 'process signal'

and 'process handle'.  The test suite would like to control the asynch/sync
execution of the interpreter during the middle of the test method, so the
CommandInterpreter::SetSynchronous(bool value) is modified to allow the mode to
be changed more than once.

In practice, it would be advisable to control the process and to set the
async/sync mode from a single thread, too.

llvm-svn: 116467