Tests: Use a testscenarios for running tests.

Instead of using load_tests to build a test suite out of a single
test case, rely on testscenarios. This library aims at running the
same test with different input data (`scenarios').

Some code is moved inside the test case to reduce the number of
scenario parameters (previously fixtures).

Change-Id: Iedb4cb3ad6341a9b397fa305b1ce2b329b6fdc15
Merge "Tests: Use a testscenarios for running tests."