Add a Closeable testing helper.

tests/helpers/__init__.py

@@ -0,0 +1,30 @@
+
+class Closeable(object):
+
+    def __init__(self):
+        self._closed = False
+
+    def __del__(self):
+        self.close()
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *args):
+        self.close()
+
+    @property
+    def closed(self):
+        return self._closed
+
+    def close(self):
+        if self._closed:
+            return
+        self._closed = True
+
+        self._close()
+
+    # implemented by subclasses
+
+    def _close(self):
+        raise NotImplementedError