try to stabilize vtc

seen in vtest:

**** v1   vsl|       1004 Begin           b bereq 1003 fetch
**** v1   vsl|       1004 VCL_use         b vcl1
**** v1   vsl|       1004 Timestamp       b Start: 1602334643.946723 0.000000 0.000000
**** v1   vsl|       1004 BereqMethod     b PUT
**** v1   vsl|       1004 BereqURL        b /2
**** v1   vsl|       1004 BereqProtocol   b HTTP/1.1
**** v1   vsl|       1004 BereqHeader     b Host: 127.0.0.1
**** v1   vsl|       1004 BereqHeader     b Content-Length: 250000
**** v1   vsl|       1004 BereqHeader     b X-Forwarded-For: 127.0.0.1
**** v1   vsl|       1004 BereqMethod     b GET
**** v1   vsl|       1004 BereqHeader     b Accept-Encoding: gzip
**** v1   vsl|       1004 BereqHeader     b X-Varnish: 1004
**** v1   vsl|       1004 VCL_call        b BACKEND_FETCH
**** v1   vsl|       1004 VCL_return      b fetch
**** v1   vsl|       1004 BackendOpen     b 26 s1 127.0.0.1 36579 127.0.0.1 60878 connect
**** v1   vsl|       1004 Timestamp       b Bereq: 1602334643.956883 0.010159 0.010159
**** v1   vsl|       1004 FetchError      b HTC eof (-1)

Ref #3433

bin/varnishtest/tests/r03433.vtc

 varnishtest "req.body and restarts"
 
-server s1 -repeat 4 {
+server s1 -repeat 2 {
+	rxreq
+	txresp -bodylen 1000
+} -start
+
+server s2 -repeat 2 {
 	rxreq
 	txresp -bodylen 1000
 } -start
@@ -14,6 +19,14 @@ varnish v1 -vcl+backend {
 		return (hash);
 	}
 
+	sub vcl_backend_fetch {
+		if (bereq.url == "/1") {
+			set bereq.backend = s1;
+		} else {
+			set bereq.backend = s2;
+		}
+	}
+
 	sub vcl_backend_response {
 		set beresp.ttl = 0.1s;
 	}