Fix issue with exec TTY caused by 15446

The bool logic around setting up the TTY ended up getting flipped
accidentally.
Also added a test for exec with TTY.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

integration-cli/docker_cli_exec_unix_test.go

@@ -42,3 +42,31 @@ func (s *DockerSuite) TestExecInteractiveStdinClose(c *check.C) {
 		c.Fatal("timed out running docker exec")
 	}
 }
+
+func (s *DockerSuite) TestExecTTY(c *check.C) {
+	dockerCmd(c, "run", "-d", "--name=test", "busybox", "sh", "-c", "echo hello > /foo && top")
+
+	cmd := exec.Command(dockerBinary, "exec", "-it", "test", "sh")
+	p, err := pty.Start(cmd)
+	c.Assert(err, check.IsNil)
+	defer p.Close()
+
+	_, err = p.Write([]byte("cat /foo && exit\n"))
+	c.Assert(err, check.IsNil)
+
+	chErr := make(chan error)
+	go func() {
+		chErr <- cmd.Wait()
+	}()
+	select {
+	case err := <-chErr:
+		c.Assert(err, check.IsNil)
+	case <-time.After(3 * time.Second):
+		c.Fatal("timeout waiting for exec to exit")
+	}
+
+	buf := make([]byte, 256)
+	read, err := p.Read(buf)
+	c.Assert(err, check.IsNil)
+	c.Assert(bytes.Contains(buf, []byte("hello")), check.Equals, true, check.Commentf(string(buf[:read])))
+}