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
    (https://github.com/test-unit/test-unit/issues/99#issuecomment-90532326).
    I did not followed all, since I think that as long as RHEL 6 is
    maintained, we might need Ruby 1.8.7 support.