Add unicode_plot constant tests

test/unicode_plot_test.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require_relative 'test_helper'
+require 'unicode_plot'
+
+# Check the UnicodePlot constants that YouPlot depends on.
+# Prepare for UnicodePlot version upgrades.
+class UnicodePlotTest < Test::Unit::TestCase
+  test 'VERSION' do
+    assert UnicodePlot::VERSION
+  end
+
+  test 'BORDER_MAP' do
+    assert_instance_of Hash, UnicodePlot::BORDER_MAP
+  end
+
+  test 'PREDEFINED_TRANSFORM_FUNCTIONS' do
+    assert_instance_of Hash, UnicodePlot::ValueTransformer::PREDEFINED_TRANSFORM_FUNCTIONS
+  end
+end