test: Track memory usage in /tmp/memwatch.log

Output looks like this:

```
HostID: cafd72ba - cat /tmp/memwatch.log

15:36:33.870    etcd    discoverd
15:36:33.883    0       0
15:36:34.902    8920    2280
15:36:35.917    19128   11088
15:36:36.932    19128   11088
15:36:37.946    19652   11612
15:36:38.962    19912   11612
15:36:39.978    19912   11612
...
```

Thanks to @heavenlyhash for the script.