Fix Ruby's test suite.
Please note that the test suite was adjusted to work with test-unit as
well as test-unit wrapper (which uses Minitest in background) as
provided by Ruby < 2.2.
Some tips were provided by original author of Ruby bindings and
test-unit maintainer in one person
I did not followed all, since I think that as long as RHEL 6 is
maintained, we might need Ruby 1.8.7 support.