Introduce load_tests mechanism for negative tests

The test loader will run the function given in the local variable
"load_test" (see http://docs.python.org/2/library/unittest.html#load-tests-protocol).
This is done after the import level and only if a valid test runner is
in place. Therefore it will solve the issue with pep8 job since
flake8 won't interpret the load_tests variable.

Change-Id: Idc160b9c83e9bd93bc4c92b7a86e36c0fc5b8a2e
Implements: bug 1291826
Merge "Introduce load_tests mechanism for negative tests"