build: Karma proxy should convert Host header
By default the Karma proxy does requests against 'localhost', however
if the target wiki server has strict host validation (e.g. multiple
virtual hosts or otherwise strictly verified) then requests may fail
since load.php is not found. An example is with MW_SERVER=devwiki.local.
MediaWiki-Vagrant and Wikimedia CI are not affected since they use
'localhost' as the virtual host.
Set Karma proxy to change the hostname accordingly.
Reference:
https://github.com/karma-runner/karma/issues/1729
https://github.com/karma-runner/karma/commit/
ae05ea4
Bug: T153757
Change-Id: I317d5686aecd1fb6cf6921cdca77670cded85607