pybind/mgr/dashboard: use setUpClass for initializeing class

pybind/mgr/dashboard: use setUpClass for initializeing class

instead of relying on __init__(), use setUpClass() to initialize class
for testing. it turns out in pytest > 4, __init__() is called for the
test class but the attributes of the instantiated class is in turn overriden.

so we have to use setUpClass to do this job.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 71979e9b46e21dc3d3cfc6f06f4a84c9b4c7ce78)

 Conflicts:
	src/pybind/mgr/dashboard/tests/test_api_auditing.py:
          Differences in import lines
	src/pybind/mgr/dashboard/tests/test_tools.py:
          Differences in import lines