completeStream changes introduced "Unconsumed" bug

[joakime]

Jetty version(s)
12.0.x and 12.1.x

Jetty Environment
core

HTTP version
HTTP/1.1

Java version/vendor (use: java -version)
Any

OS type/version
Any

Description
The new changes from issue #13470 to call completeStream only once (PR #13814) have broken RFC2616Test.test824()

The RFC2616Test.test824() is disabled in jetty-12.0.x so it didn't catch this issue.
However, the change from PR #13814 has been merged to jetty-12.1.x, which has RFC2616Test.test824() enabled.

Currently two PRs are experiencing consistent test failures due to this merge (#13690 and #13929)

We should restore/enable RFC2616Test entirely in jetty-12.0.x branch and fix this issue there, then merge that change up to jetty-12.1.x