[Tests] add test_dg_exc

This adds a command in Custom Entity that acts on the input signal.
Acting on this input signal before setting it should raise an exception
from dynamic-graph.

test_dg_exc was added to check that, and fails for now.

ref https://github.com/stack-of-tasks/dynamic-graph-python/issues/39

While here, fix exception type check in test_bindings.