ipptool: Provide a way to run a subset of tests

[michaelrsweet]

From @KurtPfeifle in ippsample issue 60:

After meanwhile quite a few ipp-1.1.test runs and being entangled in related debugging activities, I think the following would be beneficial for future testers:

• Split all the individual tests making up the IPP-1.1 test suite into separate files.
• (Re-)Create a new ipp-1.1.test suite by using INCLUDE statements for the individual files.

This way it would be more easy to run specific tests individually, especially when it comes to debugging the respective parts of the now biiiig ipp-1.1.test file.

It would also allow to make a meaningful use of ipptool's capability to test with multiple test files within one run as its usage hint suggests:

 Usage: ipptool [options] URI filename [ ... filenameN ] 

[wifiprintguy]

I'm glad you came around. 😊 The biggest issue is handling the dependency graph between DEFINEs used in a test and making sure earlier tests are also run. If they are spread across files that may make things awkward but might be fine if just done carefully.