Alternative failure mitigation of `TestExecInteractiveStdinClose`

Begin to copy the data until the command to exit and any coping to
stdin or copy from stdout/stderr has completed.
Also adding defense code to trim the possible '\x00' null value.

Signed-off-by: Dennis Chen <dennis.chen@arm.com>

integration-cli/docker_cli_exec_unix_test.go

@@ -25,10 +25,6 @@ func (s *DockerSuite) TestExecInteractiveStdinClose(c *check.C) {
 	c.Assert(err, checker.IsNil)
 
 	b := bytes.NewBuffer(nil)
-	go func() {
-		io.Copy(b, p)
-		p.Close()
-	}()
 
 	ch := make(chan error)
 	go func() { ch <- cmd.Wait() }()
@@ -36,9 +32,14 @@ func (s *DockerSuite) TestExecInteractiveStdinClose(c *check.C) {
 	select {
 	case err := <-ch:
 		c.Assert(err, checker.IsNil)
-		output := b.String()
+		io.Copy(b, p)
+		p.Close()
+		bs := b.Bytes()
+		bs = bytes.Trim(bs, "\x00")
+		output := string(bs[:])
 		c.Assert(strings.TrimSpace(output), checker.Equals, "hello")
 	case <-time.After(5 * time.Second):
+		p.Close()
 		c.Fatal("timed out running docker exec")
 	}
 }