mwconf: set $wgHTTPProxy based on wmf site

Some tests rely on an internet access, for example the Math extension
fetch material from latexml. Hacking $wgHTTPProxy does fix the issue :
https://gerrit.wikimedia.org/r/#/c/105188/

Made an anonymous function to avoid polluting the global namespace.

The default proxy is webproxy.eqiad.wmnet, it is changed for the pmtpa
cluster.

bug: 59253
Change-Id: Idb8ab2909aec3ee38ab724ca5dc2b232b1a7d267