tests: make core independent of psyneulink

Most tests came from psyneulink and used psyneulink objects despite
testing only scheduler functionality.
Replace these with generic/non-psyneulink alternatives so that
testing core graph-scheduler functionality does not depend on
psyneulink.

tests/scheduling/conftest.py

@@ -1,8 +1,27 @@
 import types
 
-import psyneulink as pnl
 import pytest
 
+import graph_scheduler as gs
+
+
+class SimpleTestNode(types.SimpleNamespace):
+    def __init__(self, name=None, is_finished_flag=True, value=0):
+        super().__init__(name=name, is_finished_flag=is_finished_flag, value=value)
+
+    def __hash__(self):
+        return id(self)
+
+    def __repr__(self):
+        return str(self.name)
+
+    def is_finished(self, execution_id):
+        return self.is_finished_flag
+
+    def add(self, n=1):
+        self.value += n
+        return self.value
+
 
 def pytest_assertrepr_compare(op, left, right):
     if isinstance(left, list) and isinstance(right, list) and op == '==':
@@ -37,12 +56,25 @@ def create_node(function=lambda x: x):
 
 
 @pytest.fixture
-def three_node_linear_composition():
-    A = pnl.TransferMechanism(name='A')
-    B = pnl.TransferMechanism(name='B')
-    C = pnl.TransferMechanism(name='C')
+def three_node_linear_scheduler():
+    A = SimpleTestNode('A')
+    B = SimpleTestNode('B')
+    C = SimpleTestNode('C')
 
-    comp = pnl.Composition()
-    comp.add_linear_processing_pathway([A, B, C])
+    sched = gs.Scheduler(graph=create_graph_from_pathways([A, B, C]))
+
+    return sched.nodes, sched
+
+
+@pytest.helpers.register
+def get_test_node():
+    return SimpleTestNode
+
+
+@pytest.helpers.register
+def run_scheduler(scheduler, func=lambda test_node: test_node, **run_kwargs):
+    for execution_set in scheduler.run(**run_kwargs):
+        for node in execution_set:
+            func(node)
 
-    return comp.nodes, comp
+    return [node.value for node in scheduler.nodes]