fix(cgpt next): Add drive name command line argument.

Skipping the system disk scan is required for testing with disk images.
fix(cgpt next): Fix fallback when the first root partition is bad.

Currently the first partition (when next_index == -1) will always be
assumed to be good because success and tries aren't checked and its
priority is then preserved. This will break fallback to a lower priority
root after the first's tries are used-up.
fix(tests): Add some basic tests for cgpt next.

Covers the basics, already found a major bug!
Merge pull request #6 from marineam/tests

Tests and bugfixes for cgpt next