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>