[System.Web] Remove unneeded namespace prefix in a test

This prefix was needed to demonstrate that the first access to AppSettings
worked while the second didn't. If we simply access the AppSettings once,
and the fix for bug BXC#11972 is applied, the test will simply pass, which
is what we care about.
[System.Web] Rename "previousSystem" to "fallback" in HttpConfigSystem

This name is a bit more clear.