Imported WPT tests often load resources using canonical absolute paths,
for example:
/html/browsers/windows/resources/message-parent.html
Our usual strategy is to rewrite such URLs to relative paths during
import so they resolve from the importing test HTML file (which also
allows for loading this HTML as a file://).
That works for same-origin relative loading with the echo server, but it
breaks down when a test constructs a cross-origin URL from an absolute
path, for example:
get_host_info().REMOTE_ORIGIN + "/some/wpt/path.html"
In that case the resource still needs to be fetched at its canonical
absolute path, so rewriting it relative to the importing document is no
longer sufficient.
Update the HTTP test server so /static/ continues to serve from the
general test root, other non-echo requests are served from the imported
WPT tree, and dynamically registered echo responses live under /echo/
to avoid path conflicts.
A test configured under LoadFromHttpServer is loaded over localhost
rather than 127.0.0.1, causing the echo server to be accessed
across origins. In general, the behavior being tested is intended
for same-origin scenarios, so update the echo server URL to use
localhost.
