Stabilize vtc

c11 tests too long a body, so it can experience a short write. If c11
fails, varnishtest terminates and, consequently, all kinds of errors
are reported, e.g. no response from s1.

src/vtc/vmod_frozen.vtc

@@ -247,6 +247,7 @@ logexpect l11 -v v1 -q "ReqURL ~ \"^/req.body\" and ReqHeader:max ~ \"^10B\"" {
 } -start
 
 client c11 {
+	non_fatal
 	txreq -url "/req.body" -hdr "max: 10B" \
 	    -nolen -hdr "Transfer-Encoding: chunked"
 	chunked "{ a: 1, b: \"hi €\\t\\\" there\", c: true, d: false, "