[System] Fixed HttpWebRequest test that sometimes failed

The TestTimeoutPropertyWithServerThatDoesntExist test sometimes failed with a 'ConnectFailure' instead of a 'Timeout'.
As per the discussion in PR #1145, the test should actually check that a ConnectFailure is thrown before the timeout is reached.
This proved unreliable with the 10.128.200.100 IP address, as it was up to the networking stack to decide whether the endpoint can be reached,
which can take quite a long time (sometimes >100secs).

Instead, we now use the loopback address with a port where no service listens, which should be equivalent.

Additionally, the assert which checks for the correct exception switched the 'expected' and 'actual' parameter, which resulted in the wrong output, this is fixed as well.