A file whose name contains a literal "%" 404s on download because the
server-built oc:downloadURL did not percent-encode the path (#2852, fixed in
reva by https://github.com/opencloud-eu/reva/pull/653). Add a scenario mirroring
the web client download flow: read the oc:downloadURL from PROPFIND and GET it
as-is (the signature in the URL authenticates it), asserting 200 and the content.
The new step "user X downloads the P file via its oc:downloadURL" lives in
WebDavPropertiesContext; a regex annotation avoids the ":" in oc:downloadURL
colliding with the turnip :placeholder syntax.
Verified red->green on posix and decomposed: red on an opencloud built on a reva
with the fix reverted (the download 404s), green on the shipped reva.
https://github.com/opencloud-eu/opencloud/issues/2852
