qapi: Pass '-u' when doing non-silent diff

qapi: Pass '-u' when doing non-silent diff

Ed-script diffs are awful compared to context diffs.  Fix another
'diff -q' while in the area (if the files are different, being
noisy makes it easier to diagnose why).

While at it, diff .err before .out, because if a test fails, .err
is more likely to contain the most important information for
fixing the failure.

Fixes: 46ec4fce
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180315125116.804342-1-eblake@redhat.com>