Remove obsolete require of 'minitest/spec'

Requiring 'minitest/autorun' is enough.
Remove unused variable
Avoid variable shadowing
Replace deprecated File.exists?
Avoid warning when replacing constant
Use new minitest test case class name
Make sure po and omf files are generated in share
Remove use of Iconv
Remove check for ancient versions of the gtk2 gem

Instead, require a more modern version in Gemfile.
Skip non-functioning tests

Since something changed in the live response from the site, either the
tests or the implementation need to be changed.