scripts/oe-selftest: Move {add,remove}_include files to case

The oe-selftest creates include files to store custom configuration to
make specific tests, every class executes a different test and may be
uses custom configuration.

So move to case class in order to simplify oe-selftest script and later
implement later a build folder per class.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>