Automatic update from web-platform-tests
[common] Correct definition of alternate host name
The value of the "alternate" host is completely at the discretion of the
server operator (via the `alternate_hosts` configuration). It is not
guaranteed to have any resemblance to the primary host name. Despite
this, the shared `get-host-info.sub.js` file attempted to derive the
alternate host name in terms of the primary host name.
Replace that faulty derivation with the value which is present in the
server configuration.
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wpt-commits: 08b42563fba41573b55c8e5f2136d6c8bc16e7df
wpt-pr: 17873
