tests/int: fix TestHook flakiness

Since commit 3cdda46 the poststart hooks runs after the container
process start, and so they race.

Move the poststart hook check to a separate step after the container
process has exited.

Fixes 5245.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/exec_test.go

@@ -1075,11 +1075,14 @@ func TestHook(t *testing.T) {
 	container, err := newContainer(t, config)
 	ok(t, err)
 
-	// e.g: 'ls /prestart ...'
 	var cmd strings.Builder
 	cmd.WriteString("ls ")
 	for _, hook := range hookFiles {
-		cmd.WriteString("/" + hook + " ")
+		// The poststart hook is racing with this ls command (run as the
+		// container init), so we don't check that hook worked here yet.
+		if hook != "poststart" {
+			cmd.WriteString("/" + hook + " ")
+		}
 	}
 
 	var stdout strings.Builder
@@ -1098,6 +1101,13 @@ func TestHook(t *testing.T) {
 	// Wait for process
 	waitProcess(&pconfig, t)
 
+	// Check that poststart hook worked.
+	f, err = os.Open(config.Rootfs + "/poststart")
+	if err != nil {
+		t.Fatalf("poststart hook failed: %s", err)
+	}
+	f.Close()
+
 	if err := container.Destroy(); err != nil {
 		t.Fatalf("container destroy %s", err)
 	}