stabilize the error-during-streaming test

the delay is not an optimal solution, a barrier would be much more
appropriate, but I have not found the right place

src/tests/errors.vtc

@@ -8,6 +8,7 @@ varnishtest "test error handling"
 server s1 {
 	rxreq
 	txresp -nolen -hdr "Content-Encoding: br" -hdr "Content-Length: 4"
+	delay .2
 	sendhex "de ad be ef"
 	expect_close
 } -start
@@ -27,11 +28,17 @@ varnish v1 -arg "-p vsl_mask=+VfpAcct" -vcl+backend {
 client c1 {
 	txreq
 	rxresphdrs
+	expect resp.status == 200
 	expect_close
 } -run
 
 server s1 -wait
-server s1 -start
+server s1 {
+	rxreq
+	txresp -nolen -hdr "Content-Encoding: br" -hdr "Content-Length: 4"
+	sendhex "de ad be ef"
+	expect_close
+} -start
 
 varnish v1 -vcl+backend {
 	import ${vmod_brotli};