Make TestMultipleVMs_Isolated stable by not sharing its ttrpc client

TestMultipleVMs_Isolated is still unstable (see #581). Apparently having
multiple simultaneous requests from the same client is known to be
problematic (see containerd/ttrpc#72).

This commit workarounds the issue by making a client per VM.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

Author: kzys

runtime/service_integ_test.go

@@ -305,9 +305,6 @@ func TestMultipleVMs_Isolated(t *testing.T) {
 	image, err := alpineImage(testCtx, client, defaultSnapshotterName)
 	require.NoError(t, err, "failed to get alpine image")
 
-	fcClient, err := newFCControlClient(containerdSockPath)
-	require.NoError(t, err, "failed to create fccontrol client")
-
 	cfg, err := config.LoadConfig("")
 	require.NoError(t, err, "failed to load config")
 
@@ -357,6 +354,11 @@ func TestMultipleVMs_Isolated(t *testing.T) {
 				MachineCfg: &proto.FirecrackerMachineConfiguration{MemSizeMib: 1024},
 			}
 
+			fcClient, err := newFCControlClient(containerdSockPath)
+			if err != nil {
+				return err
+			}
+
 			resp, createVMErr := fcClient.CreateVM(ctx, req)
 			if createVMErr != nil {
 				matches, err := findProcess(ctx, findFirecracker)