Amend python_destructor_exception runtime test

Suppress the message that PyErr_WriteUnraisable writes to stderr,
but check that it is called by checking some of the expected message contents.
The output varies slightly for different versions of Python and -builtin

Examples/test-suite/python/python_destructor_exception_runme.py

@@ -2,13 +2,31 @@ import python_destructor_exception
 from StringIO import StringIO
 import sys
 
-#buffer = StringIO()
-#sys.stderr = buffer
-
-attributeErrorOccurred = False
-try:
+def error_function():
     python_destructor_exception.ClassWithThrowingDestructor().GetBlah()
-except AttributeError, e:
-    attributeErrorOccurred = True
 
-assert attributeErrorOccurred
+def runtest():
+    attributeErrorOccurred = False
+    try:
+        error_function()
+    except AttributeError, e:
+        attributeErrorOccurred = True
+    return attributeErrorOccurred
+
+def runtestcaller():
+    stderr_saved = sys.stderr
+    buffer = StringIO()
+    attributeErrorOccurred = False
+    try:
+        # Suppress stderr while making this call to suppress the output shown by PyErr_WriteUnraisable
+        sys.stderr = buffer
+
+        attributeErrorOccurred = runtest()
+    finally:
+        sys.stderr.flush()
+        sys.stderr = stderr_saved
+
+    assert attributeErrorOccurred
+    assert buffer.getvalue().count("I am the ClassWithThrowingDestructor dtor doing bad things") >= 1
+
+runtestcaller()