REGTESTS: fix the race conditions in secure_memcmp.vtc

In the same way than for normalize_uri.vtc, a "Connection: close" header is
added to all responses to avoid any connection reuse. This should avoid any
"HTTP header incomplete" errors.
diff --git a/reg-tests/converter/secure_memcmp.vtc b/reg-tests/converter/secure_memcmp.vtc
index 67a7b7d..6ff74e6 100644
--- a/reg-tests/converter/secure_memcmp.vtc
+++ b/reg-tests/converter/secure_memcmp.vtc
@@ -7,12 +7,12 @@
 
 server s1 {
 	rxreq
-	txresp
+	txresp -hdr "Connection: close"
 } -repeat 4 -start
 
 server s2 {
 	rxreq
-	txresp
+	txresp -hdr "Connection: close"
 } -repeat 7 -start
 
 haproxy h1 -conf {