Core: preprocess out TestPromptFormats

TestPromptFormats appears as though it may be a useful unit test.
Unfortunately, there is no invocation mechanism in place right now.  It is
unclear how to add a unit test for this scenario to the existing tests.  It
would be ideal to remove this entirely, but I am hopeful that this can/will be
pulled out into a test still since it uses a user accessible interface.

llvm-svn: 204309