Scanning: Test suite/driver

[michaelrsweet]

Add a test driver and end-to-end test functions to the testpappl program:

• [ ] Add pwg-scanner.c in the testsuite directory, patterned on pwg-driver.c; ideally the driver should return test images with different colors/patterns as appropriate, and auto-associate to
• [ ] Update pwg-driver.c to include a new "pwg_mfp" driver that auto-associates with the test scanner driver.
• [ ] Update testpappl.c to add a "scan" test to validate scanning.
• [ ] Update testmainloop.c to call the new scanner APIs prior to papplMainloop.