Support CIDR ranges in $wgSquidServersNoPurge

Use IP::isInRange() in wfIsConfiguredProxy() to allow matching against
CIDR entries in $wgSquidServersNoPurge. This will allow maintainers of
large networks to whitelist contiguous blocks of IPv4 and/or IPv6
addresses as trusted X-Forwarded-For providers.

This change also makes a small change to
WebRequestTest::testGetIpLackOfRemoteAddrThrowAnException() which was
failing under some configurations due to non-default globals
configuration.

Bug: 52829
Change-Id: I49e34bdf13e8e8c6cd169c362c283fe1034bdc6d
(cherry picked from commit f111b2687c894bd744f53edff4ae049ddb48c59a)