Add revised wait for the WiFi in FTU tests

I discovered that the FTU app sets the `wifi.enabled` setting upon startup however on the Leo device sometimes the test can reach the WiFi step before the `mozWifiManager` is actually enabled and thus no networks are found. In a real life scenario the user can tap 'refresh' to get a new list.

As these two tests are tagged with `wifi` the wait is reasonable.

I've also updated the `is_wifi_enabled` property in the GaiaData class to represent the real enabled state of the mozWifiManager so it ought to be a bit more trustworthy now.

This is possibly occasionally the same error we see on the CI/Unagi which has been difficult to replicate locally.

NB this patch is for master but I have been testing it on v1-train builds (Unagi and Leo) without problem.

`master` -> ja ja
`v1-train` -> ja ja