build: Karma proxy should convert Host header

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