Support running tests from directories outside the test root.

In this design each directory containing tests gets a metadata
directory and is mounted somewhere under /. So one might have a
set of local tests that are waiting to be upstreamed with a folder
structure like

local
  tests
  metadata

Alongside the web-platfrom-tests directory that might look like

wpt
  tests
  meta

web-platform-tests would be mounted at / and local at /_local/.

This could be specified in a config.ini file as

[manifest:upstream]
tests = wpt/tests
metadata = wpt/meta
url_base = /

[manifest:local]
tests = local/tests
metadata = local/meta
url_base = /_outbound/