It was apparently possible to get a propertiesChanged event from an
earlier test case with an empty Groups property. That ended up this case
exiting immediately before running through the steps and consequently,
failing due to missed operations. Make this less likely to happen by
accepting the Groups property emptying event only after a group has been
added for a peer first.
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>