tests: Use error_free_or_abort() where appropriate

Done with this Coccinelle semantic patch:

    @@
    expression E;
    @@
    -    g_assert(E);
    -    error_free(E);
    +    error_free_or_abort(&E);

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487362554-5688-1-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>