tests: Clean up test-string-output-visitor

Use &error_abort and error_free_or_abort() in more places, use
the generated qapi_free_intList() instead of open-coding it,
reduce the scope of some variables, avoid code duplication
during test setup with visitor_output_setup_internal(), and
copy the visitor_reset() concept from the qmp-output test to
the string-output test.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465490926-28625-11-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>