[qa] adds a rather stupid test to check valueholder works correctly. Still some operations missing.

bonobo/util/testing.py

@@ -1,3 +1,4 @@
+from contextlib import contextmanager
 from unittest.mock import MagicMock
 
 from bonobo.execution.node import NodeExecutionContext
@@ -7,3 +8,12 @@ class CapturingNodeExecutionContext(NodeExecutionContext):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.send = MagicMock()
+
+
+@contextmanager
+def optional_contextmanager(cm, *, ignore=False):
+    if cm is None or ignore:
+        yield
+    else:
+        with cm:
+            yield