Tests: todo_alerts() with sendfile for added mp4 overflow test.

mp4.t

diff --git a/mp4.t b/mp4.t
index d5476ee..2cca40b 100644
--- a/mp4.t
+++ b/mp4.t
@@ -124,6 +124,9 @@
 
 }
 
+$t->todo_alerts() if $t->read_file('nginx.conf') =~ /sendfile on/
+	and !$t->has_version('1.17.9');
+
 ###############################################################################
 
 sub durations {