fix(test): avoid port collision between master gRPC and volume ports

AllocateMiniPorts(1) reserved masterPort and masterPort+GrpcPortOffset
by holding listeners open, but closed them on return. The subsequent
AllocatePorts call bound 127.0.0.1:0, so the OS could immediately reuse
the just-released mini gRPC port as a volume port — causing the volume
server to fail at bind time with "address already in use".

Introduce AllocatePortSet(miniCount, regularCount) that holds every
listener open until the full set is chosen, and route the five volume
test cluster builders through it.

test/volume_server/framework/cluster_mixed.go

@@ -93,22 +93,17 @@ func StartMixedVolumeCluster(t testing.TB, profile matrix.Profile, goCount, rust
 		t.Fatalf("write security config: %v", err)
 	}
 
-	miniPorts, err := testutil.AllocateMiniPorts(1)
-	if err != nil {
-		t.Fatalf("allocate master port pair: %v", err)
-	}
-	masterPort := miniPorts[0]
-	masterGrpcPort := masterPort + testutil.GrpcPortOffset
-
 	// 2 ports per server (admin, grpc); add 1 more when public port is split out.
 	portsPerServer := 2
 	if profile.SplitPublicPort {
 		portsPerServer = 3
 	}
-	ports, err := testutil.AllocatePorts(total * portsPerServer)
+	miniPorts, ports, err := testutil.AllocatePortSet(1, total*portsPerServer)
 	if err != nil {
-		t.Fatalf("allocate volume ports: %v", err)
+		t.Fatalf("allocate ports: %v", err)
 	}
+	masterPort := miniPorts[0]
+	masterGrpcPort := masterPort + testutil.GrpcPortOffset
 
 	isRust := make([]bool, total)
 	for i := goCount; i < total; i++ {