silenttestrunner: add environment variable to make tests noisy again

As I've been working on complicated extension code it's been handy to
be able to get standard unittest verbose output so I can find crashers
more efficiently.

tests/silenttestrunner.py

@@ -1,4 +1,4
-import unittest, sys
+import unittest, sys, os
 
 def main(modulename):
     '''run the tests found in module, printing nothing when all tests pass'''
@@ -16,3 +16,6 def main(modulename):
             print
             sys.stdout.write(exc)
         sys.exit(1)
+
+if os.environ.get('SILENT_BE_NOISY'):
+    main = unittest.main