daemon: parted: part-get-name: switch from sgdisk to parted (RHBZ#1088424).

Use parted to get the name of partitions in GPT layouts instead of
sgdisk, to reduce the possible discrepancy between output of tools.

The actual case here is that recent parted versions fixed/improved their
UTF-16 handling of partition names in GPT, and sgdisk seems to not be
properly handling them, returning also unicode control bytes.
Since parted can provide partition names already, just make use of it.

Since sgdisk is no more needed for part_get_name, the function is no
more optional on it.
ruby: tests: isolate boilerplate in common file

Isolate in a common file all the standard boilerplate in tests, i.e. the
import of the test framework and the guestfs module (including the
import path hack needed for the latter).

Thanks to Vít Ondruch for the precious hints and suggestions.
ruby: tests: convert from Test::Unit to MiniTest (RHBZ#1085029)

Convert the tests to the MiniTest test framework, but keeping the usage
of the old Test::Unit as fallback in case MiniTest is not available.
In the latter case, use a bit of "glue" to make the old API look like
the new API, so we can just rely on the newer MiniTest API.