[WifiIOTConnectionTest] Prevent use a single scan result in a test

Currently, all conneciton interations use a same single scan result
to decide AP existed or not. It would cause test failures because
the test environment is a dense environment. Do wifi scans before
each interation to prevent this situation.

Bug: 254570241
Test: Verified locally
Change-Id: I7af89ffa59744fadba4aca8a1e71da9663e82bd5