Create fake "smartctl --json --scan" for testing

Fake data can be fed to `smartctl_exporter` by using the `--smartctl.fake-data` flag which reads the JSON from `./testdata` for devices. `readSMARTctlDevices()` exec's `smartctl --json --scan` to get the list of devices. It would be nice to provide "fake" scan data to use for testing purposes.