For inspiration, there were used some interesting approaches to execute
the Minitest 4.x testsuite against Minitest 5.x:
http://pkgs.fedoraproject.org/cgit/rubygem-idn.git/tree/rubygem-idn.spec
http://pkgs.fedoraproject.org/cgit/rubygem-raindrops.git/tree/rubygem-rai...
But still, please discuss this topic with upstream preferably, since in
Ruby 2.2, there might not be shipped any testing framework:
https://bugs.ruby-lang.org/issues/9711
Vít