testing/testcase: snapshot *.cfg and .*.cfg files in instance dir

Keeping buildout.cfg and .installed.cfg can help diagnosing test
failures.

slapos/testing/testcase.py

@@ -295,6 +295,8 @@ class SlapOSInstanceTestCase(unittest.TestCase):
       '*/etc/*',
       '*/var/log/*',
       '*/.*log',
+      '*/.*cfg',
+      '*/*cfg',
   )
 
   # Methods to be defined by subclasses.