fix some ineffectual assignments

to make goreportcard a bit happier
https://goreportcard.com/report/github.com/docker/docker

also found that `TestCpToErrDstParentNotExists()` was
partially broken, because a `runDockerCp()` was inadvertently
removed in f26a31e80c

`TestDaemonRestartSaveContainerExitCode()` didn't verify
the actual _Error_ message, so added that to the test,
and updated the test to take into account that the
"experimental" CI enables `--init` on containers.

`TestVolumeCLICreateOptionConflict()` only checked
for an error to occur, but didn't validate if the
error was due to conflicting options.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration-cli/docker_cli_build_unix_test.go

@@ -140,6 +140,8 @@ func (s *DockerSuite) TestBuildCancellationKillsSleep(c *check.C) {
 	buildCmd.Dir = ctx.Dir
 
 	stdoutBuild, err := buildCmd.StdoutPipe()
+	c.Assert(err, checker.IsNil)
+
 	if err := buildCmd.Start(); err != nil {
 		c.Fatalf("failed to run build: %s", err)
 	}