tests: Don't try to collect tests from Agent helper classes

resolves:

tests/test_agent.py:12
  /home/runner/work/mesa/mesa/tests/test_agent.py:12: PytestCollectionWarning: cannot collect test class 'TestAgent' because it has a __init__ constructor (from: tests/test_agent.py)
    class TestAgent(Agent):
tests/test_agent.py:20
  /home/runner/work/mesa/mesa/tests/test_agent.py:20: PytestCollectionWarning: cannot collect test class 'TestAgentDo' because it has a __init__ constructor (from: tests/test_agent.py)
    class TestAgentDo(Agent):

tests/test_agent.py

@@ -9,6 +9,7 @@
 from mesa.model import Model
 
 
+@pytest.mark.skip(reason="Helper class, not a test class")
 class TestAgent(Agent):
     """Agent class for testing."""
 
@@ -17,6 +18,7 @@ def get_unique_identifier(self):
         return self.unique_id
 
 
+@pytest.mark.skip(reason="Helper class, not a test class")
 class TestAgentDo(Agent):
     """Agent class for testing."""