lib/oeqa: add oeTest superclass

lib/oeqa: add oeTest superclass

Add oeTest superclass, make oeRuntimeTest inherit that and
make the necesarry adjustments. Tests in lib/oeqa/runtime don't
change, they still inherit oeRuntimeTest.
We should do this because oetest.py in the future can be a base module
for more stuff than oeRuntimeTest.

(From OE-Core rev: cd4ed41a070bd52c446ac3df8337f17ab9421145)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>