Switch TestCase attribute criteria to result

Switch TestCase attribute criteria to result

It mainly avoids mixing input and output.
Criteria is now an input set in functest/ci/testcases.yaml and then
must be passed as __init__() args (which will be proposed in an
additional change).

Then it also renames the related TestCase method to
check_result().

Change-Id: Ifc3c8e3ea6cde7e3edf7174bed4bf2bf0894e8e3
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>