Fix broken test: com.android.settings.wifi.WifiSettingsTest.
PasspointConfiguration objects require both HomeSp and Credential
objects initialized, and since today it is enforced in the Wi-Fi API.
Updated the test mocks to initialize the Credential object which was
missing.
Bug: 149175146
Test: make RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.wifi.WifiSettingsTest
Change-Id: I709a6bc104141ac88b2c88bdac4f3a9a3d296ec9