REGTEST: Don't use the websocket to validate http-check

Now, some conformance tests are performed when an HTTP connection is
upgraded to websocket. This make the http-check-send.vtc script failed for
the backend <be6_ws>. Because the purpose of this health-check is to pass a
"Connection: Upgrade" header on an http-check send rule, we may use a dummy
protocal instead.
diff --git a/reg-tests/checks/http-check-send.vtc b/reg-tests/checks/http-check-send.vtc
index 3c07e1c..9773805 100644
--- a/reg-tests/checks/http-check-send.vtc
+++ b/reg-tests/checks/http-check-send.vtc
@@ -72,11 +72,11 @@
     expect req.proto == HTTP/1.1
     expect req.http.host == "ws-host"
     expect req.http.connection == "upgrade"
-    expect req.http.upgrade == "websocket"
+    expect req.http.upgrade == "raw-proto"
     txresp \
       -status 101 \
       -hdr "connection: upgrade" \
-      -hdr "upgrade: websocket"
+      -hdr "upgrade: raw-proto"
 } -start
 
 
@@ -154,7 +154,7 @@
 
     backend be6_ws
         log ${S6_addr}:${S6_port} len 2048 local0
-        http-check send meth GET uri / ver HTTP/1.1 hdr host ws-host hdr connection upgrade hdr upgrade websocket
+        http-check send meth GET uri / ver HTTP/1.1 hdr host ws-host hdr connection upgrade hdr upgrade raw-proto
         http-check expect status 101
         server srv ${s6_addr}:${s6_port} check inter 200ms rise 1 fall 1