ipptool: Add support for running sub-tests using xxx-supported values

[michaelrsweet]

It would be useful to have ipptool support iteration over a series of supported values reported by the printer, e.g.:

EXPECT foo-supported OF-TYPE keyword TEST-VALUES variable "testfile"

or something like that, where ipptool would define the specified variable and then include the specified test file to perform additional tests.

[michaelrsweet]

Since we'd likely want to implement a test matrix from multiple attributes, the syntax should probably be something like:

EXPECT foo-supported OF-TYPE keyword TEST-VALUES foo-variable
EXPECT bar-supported OF-TYPE resolution TEST-VALUES bar-variable
TEST-SCRIPT "filename.test"

[michaelrsweet]

Tracking as libcups issue 11...