public void assertMergeOneReply(String recipient) {
        assertSelect(List.of(recipient));

        Map<String, Integer> replies = new HashMap<>();
        replies.put(recipient, ErrorCode.NONE);
        assertMergeOk(replies, List.of(recipient));

        replies.put(recipient, ErrorCode.TRANSIENT_ERROR);
        assertMerge(replies, List.of(ErrorCode.TRANSIENT_ERROR), List.of(recipient));
    }
Ensures that the current setup chooses a single recipient, and that it merges similarly to how the
{@link DocumentProtocol} would merge these.

@param recipient The expected recipient.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/protocol/test/PolicyTestFrame.java