oeqa/selftest/case: Migrate case class to the new OEQA framework

Summary of the changes:

- Use OETestCase as base class instead of unittest.TestCase
- Remove LogResults decorator the new framework provides logging into
  the core functionality.
- Logger is now self.logger instead of self.log
- Move comments into docstrings in several help methods
- Use get_test_layer() method instead of access monkey patched variable
  in old oeSelfTest case class.

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