TestExecStartFails is broken on Windows

See [this log](https://jenkins.dockerproject.org/job/Windows-PRs/14229/console) for example:

```
FAIL: docker_cli_exec_test.go:616: DockerSuite.TestExecStartFails

docker_cli_exec_test.go:624:
    c.Assert(out, checker.Contains, "executable file not found")
... value string = ""
... substring string = "executable file not found"
```
