user_test.rb

class ActiveSupport::TestCase 
  class << self
    remove_method :describe

    alias :context :describe
  end

  extend MiniTest::Spec::DSL
end

class UserTest < ActiveSupport::TestCase

  def setup
  end

  context '....' do
    test '....' do

    end
  end

  test '....' do

  end
end

You should remove ActiveSupport::TestCase.describe before you extend MiniTest::Spec::DSL, as you have done. But you need to do the alias after the extend to make sure you are aliasing the right method.

I assume you are working off this post to enable the spec DSL in rails?