WifiNetworkSpecifierTest: Add scan retry

If the first scan does not find the matching bssid for the current
network, retry 2 more times before declaring failures. Wifi scans by
nature are flaky, so we should retry before asserting.

Bug: 156712429
Test: atest WifiNetworkSpecifierTest
Change-Id: I4be92e1b16d9d3fc38b40e3eb62c4f90a21fe2ed