tests: Stabilize fellow_coverage.vtc

src/vtc/fellow_coverage.vtc

@@ -25,12 +25,15 @@ varnish v1 -errvcl {can only be used on a fellow storage} {
 	}
 }
 
+barrier b1 cond 3
+
 server s1 {
 	rxreq
 	expect req.url == "/256k"
 	txresp -bodylen 262144
 	rxreq
 	expect req.url == "/256k+1"
+	barrier b1 sync
 	txresp -bodylen 262145
 } -start
 
@@ -47,6 +50,7 @@ client c1 {
 	expect resp.bodylen == 262144
 
 	txreq -url "/256k+1"
+	barrier b1 sync
 	rxresp
 	expect resp.status == 503
 } -start
@@ -58,6 +62,7 @@ client c2 {
 	expect resp.bodylen == 262144
 
 	txreq -url "/256k+1"
+	barrier b1 sync
 	rxresp
 	expect resp.status == 503
 } -start