Provided a mechanism for the test class to cleanup after itself once it's done.

This will remove the confusion experienced when previous test runs left some
files (both intermediate or by-product as a result of the test).

lldbtest.TestBase defines a classmethod tearDownClass(cls) which invokes the
platform-specific cleanup() function as defined by the plugin; after that, it
invokes a subclass-specific function classCleanup(cls) if defined; and, finally,
it restores the old working directory.

An example of classCleanup(cls) is in settings/TestSettings.py:

    @classmethod
    def classCleanup(cls):
        system(["/bin/sh", "-c", "rm output.txt"])

where it deletes the by-product "output.txt" as a result of running a.out.

llvm-svn: 114058