Prepare a test skeleton

test/test_helper.rb

@@ -1,4 +1,4 @@
 $LOAD_PATH.unshift File.expand_path('../lib', __dir__)
 require 'uplot'
 
-require 'minitest/autorun'
+require 'test/unit'

test/uplot_test.rb

@@ -1,11 +1,7 @@
 require 'test_helper'
 
-class UplotTest < Minitest::Test
+class UplotTest < Test::Unit::TestCase
   def test_that_it_has_a_version_number
-    refute_nil ::Uplot::VERSION
-  end
-
-  def test_it_does_something_useful
-    assert false
+    assert_kind_of String, ::Uplot::VERSION
   end
 end