Eliminate some deprecation warnings.
Eliminate some deprecation warnings.

Minitest 5 uses `Minitest::Test` as test base class, while Minitest 4
uses `MiniTest::Unit::TestCase`. Using the Minitest 4 class in a
Minitest 5 environment yields a lot of deprecation messages in
the form of

    MiniTest::Unit::TestCase is now Minitest::Test. From /home/dm/code/slim/test/core/test_code_escaping.rb:3:in `(root)'

to $stdout/$stderr.

XueQian/slim,zakuro9715/slim,raxoft/slim,Nikeshsuwal/slim,slim-template/slim,etdev/slim,slim-template/slim,cheunghy/slim,ruijsun/slim,ericboehs/slim,metallurgical/slim,Nikeshsuwal/slim,ericboehs/slim,fly19890211/slim,fly19890211/slim,zakuro9715/slim,sideci-sample/sideci-sample-slim,JuanitoFatas/slim,cheunghy/slim,XueQian/slim,mojavelinux/slim,raxoft/slim,slim-template/slim,JuanitoFatas/slim,mojavelinux/slim,ruijsun/slim,etdev/slim,metallurgical/slim