wifi-manger: Fix crash on shutdown

I was wondering why CI failed for !1356 (merged) and not before. #1025 (closed) has the details.

While looking at the cause i noticed

• the screenshot tests were dependent on available services so I added a patch for this
• made some consistency improvements to the Wi-Fi code
  • use Wi-Fi consistently
  • make it a bit more obvious if we're handling the dev tracking wi-fi networks or the one tracking the connection
  • Since we don't have outstanding MRs in that area: rename the source files to be consistent with our current naming patterns

/cc @arun-mani-j does that look correct to you? If so we shoudln't wait too long with merging to ensure people can run without issues.