oe-selftest: New object SStateBase cut off from SStateTests.

- SStateBase object contains basic methods used to run sstate related tests
- SStateTests now contains only sstate-related tests

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
oe-selftest: renamed sstate.py module to sstatetests.py

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
oe-selftest: separated the SStateBase and SStateTests in different modules

- SStateBase now has its own module to be imported by itself by other modules like sstatetests.py

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>