wifi: Use "wifi-" prefix for all test-suites and examples

The goal of this MR is to use the prefix wifi- for all wifi related test-suites and examples. This is an item that has been raised during WNS3 when comparing wifi module to lte module. Indeed having such naming convention will enable easier wildcarding.

By the way @tomhenderson how can we perform such wildcarding? I don't think it's documented anywhere...