tests: disable logging on stderr

When running tests we don't want to clutter the output with logs.
Environment variable SLAPOS_TEST_DEBUG can be set to restore the
verbosity

slapos/tests/__init__.py

@@ -30,9 +30,9 @@ import logging
 import os
 
 # When running with debug enabled install Ctrl+C handler and output more logs,
-# otherwise disable warning level logs.
+# otherwise disable log output.
 if os.environ.get('SLAPOS_TEST_DEBUG'):
   unittest.installHandler()
-  logging.basicConfig(level=logging.WARNING)
+  logging.basicConfig(level=logging.DEBUG)
 else:
-  logging.disable(logging.ERROR)
\ No newline at end of file
+  logging.basicConfig(filename=os.devnull)