test: py: Use global pytestmark for hush tests

All tests in test_hush_if_test depends on hush parser to be
present. This patch simplify test dependencies by using global
pytestmark.

Reported-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Stephen Warren <swarren@nvidia.com>