Refactor test for better error messages

tests/functional/beam_tracing/test_forests.py

 import finesse.tracing.ctracer as ctracer
 
 
-def trees_equal(t1, t2):
-    if t1 is None and t2 is None:
-        return True
+def assert_trees_equal(t1, t2):
+    # Both t1 and t2 must be None or not None.
+    if t1 is None or t2 is None:
+        # Nothing to test.
+        return
+    elif (t1 is None and t2 is not None) or (t1 is not None and t2 is None):
+        assert False
 
-    if t1.node != t2.node or t1.dependency != t2.dependency:
-        return False
+    assert t1.node == t2.node, f"{t1} node {t1.node} != {t2} node {t2.node}"
+    assert (
+        t1.dependency == t2.dependency
+    ), f"{t1} dependency {t1.dependency} != {t2} dependency {t2.dependency}"
 
-    return trees_equal(t1.left, t2.left) and trees_equal(t1.right, t2.right)
+    assert_trees_equal(t1.left, t2.left)
+    assert_trees_equal(t1.right, t2.right)
 
 
 def test_fp_cavity_forest(model):
@@ -41,7 +48,7 @@ def test_fp_cavity_forest(model):
     assert len(forest) == len(model._trace_forest.forest)
 
     for tree, ttree in zip(forest, model._trace_forest.forest):
-        assert trees_equal(tree, ttree)
+        assert_trees_equal(tree, ttree)
 
 
 def test_fp_cavity_with_laser_gauss_forest(model):
@@ -79,7 +86,7 @@ def test_fp_cavity_with_laser_gauss_forest(model):
     assert len(forest) == len(model._trace_forest.forest)
 
     for tree, ttree in zip(forest, model._trace_forest.forest):
-        assert trees_equal(tree, ttree)
+        assert_trees_equal(tree, ttree)
 
 
 def test_mfp_forest(model):
@@ -143,7 +150,7 @@ def test_mfp_forest(model):
     assert len(forest) == len(model._trace_forest.forest)
 
     for tree, ttree in zip(forest, model._trace_forest.forest):
-        assert trees_equal(tree, ttree)
+        assert_trees_equal(tree, ttree)
 
 
 # TODO (sjr) Lots more potential configurations but main things to focus on are: