REGTESTS: fix the race conditions in iff.vtc

A "Connection: close" header is added to responses to avoid any connection
reuse. This should avoid any "HTTP header incomplete" errors.
diff --git a/reg-tests/converter/iif.vtc b/reg-tests/converter/iif.vtc
index ffd58ee..f412daf 100644
--- a/reg-tests/converter/iif.vtc
+++ b/reg-tests/converter/iif.vtc
@@ -5,7 +5,7 @@
 
 server s1 {
 	rxreq
-	txresp
+	txresp -hdr "Connection: close"
 } -repeat 3 -start
 
 haproxy h1 -conf {