oeqa/selftest/cases: Migrate test cases into the new oe-qa framework

New framework has different classes/decorators so adapt current test cases to
support these. Changes include changes on base classes and decorators.

Also include paths in selftest/__init__.py isn't needed because the
loader is the standard unittest one.

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