QPrinter - Clean up Print Engine Key defaults and tests

The PrintEngine keys are not consistently treated across the platforms
and are not properly tested.  Start the process of making the print
engines behave consistently by documenting and testing the current
behavior. Ensure all unsupported features return a consistent
default value.

The auto test for valuePreservation() has been flaky depending on the
platform and installed printers so remove it and replace it with more
complete testing.  If no native printers available then don't test
the native engines.

Fixes for the individual inconsistent keys will follow.

Task-number: QTBUG-26430

Change-Id: Iab914d7e0a5ae4a2cdc24c8645751f0910cf440c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

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