ipptool: Add DISPLAY-WARNING predicate

[michaelrsweet]

It would be nice to support warnings when running tests, e.g. if a Printer supports a deprecated operation or attribute, we could write:

EXPECT operations-supported WITH-VALUE 0x0012 DISPLAY-WARNING "The Purge-Jobs operation is DEPRECATED and SHOULD NOT be implemented any longer."

Similarly, we could display a warning for a status code that is unexpected:

STATUS client-error-not-found DISPLAY-WARNING "client-error-not-found is not defined for this operation."