fix TestParseRunVolumes with Go 1.3 randomization

This PR is required for #6520.

It fixes the following failure encountered with #6520:
```
--- FAIL: TestParseRunVolumes (0.00 seconds)
        config_test.go:106: Error parsing volume flags, `-v /hostTmp:/containerTmp:ro -v /hostVar:/containerVar:rw` should mount-bind /hostTmp into /containeTmp and /hostVar into /hostContainer. Received [/hostVar:/containerVar:rw /hostTmp:/containerTmp:ro]
```