patchtest-send-results: Add 'References' header

patchtest-send-results: Add 'References' header

Based on https://www.rfc-editor.org/rfc/rfc2822#section-3.6.4, to get
proper threading there should be a 'References' field defined in
Patchtest's response headers if its replies are going to thread properly
on Patchwork. This wasn't there before, hence why were were seeing the
responses in the right place on the mailing list but not Patchwork.

Since we only want Patchtest to reply directly to the tested patch,
the 'References' field should only need to contain the same ID as the
'In-Reply-To' one.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>