[Piglit] Replace dmesg functions in exectest with dmesg class

[Dylan Baker]

This series introduces a new module with a different approach for
working with dmesg, and 21 tests for that class.

There are several advantages to this approach as a whole:
1) It isolates the dmesg code better. A single module containing all of
   the dmesg logic, and only two calls needed outside of the module make
   it easy to use, and far less invasive
2) It eliminates the need for lots of if checks. By providing a dummy
   version of the same class when checking dmesg isn't requested or
   appropriate there is no need to constantly check if the run is
   checking dmesg.