base.py: add assertExists and assertNotExists to oeselftest
Current osselftest print confusing assertion message when using
self.assertTrue(os.path.exists(filepath)) to test file path,
example of confusing assertion message:
AssertionError: False is not true
Add the assertExists and assertNotExists to improve assertion
message and simplify coding, using selft.assertExists(filepath),
will print meaningful assertion message:
AssertionError: <filepath> does not exist
[YOCTO #11356]
Signed-off-by: Yeoh Ee Peng <ee.peng.yeoh@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
