RouteTest.trailer does not take into account attribute

We currently have two ways to set a trailing header on the response:
* For HTTP/1.1 and HTTP/2 alike, set it in the LastChunk of a Chunked response
* For HTTP/2 only, use the `trailer` attribute

When testing, `RouteTest.trailer` will only show the trailer from the LastChunk, not from the attribute. We could consider exposing the trailer - though it could also incorrectly give the impression that that'd work on HTTP/1.1?

Start at RouteTest.trailer and trace how it reads the response trailer from the LastChunk. Compare that behavior with the HTTP/2 trailer attribute described in the issue, then identify the expected distinction between HTTP/1.1 and HTTP/2 and add coverage showing the intended result.

Written by the indexing model from the issue text.