Fix flaky test case of TestExecInteractiveStdinClose

This issue has been reported by issue #36877.

The purpose of this test case is for the regression test of #12546,
so we only need to make sure the essential of the testing is still
in the way to check that while not disturbed by some testing noises,
which is exactly what this PR want to do.

Signed-off-by: Dennis Chen <dennis.chen@arm.com>
Upstream-commit: 96abf9f59ebd350cc9a70034b4d30279e6ae04e4
Component: engine

components/engine/integration-cli/docker_cli_exec_unix_test.go

@@ -33,7 +33,9 @@ func (s *DockerSuite) TestExecInteractiveStdinClose(c *check.C) {
 	select {
 	case err := <-ch:
 		c.Assert(err, checker.IsNil)
-		output := b.String()
+		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):
 		c.Fatal("timed out running docker exec")