Add tests for REXML::Text.check

This patch will add missing REXML::Text.check tests.
This is the tests for the part that is checked using a regular expression.
https://github.com/ruby/rexml/blob/b2ec329dc1dc7635b224a6d61687c24b1e1db6fd/lib/rexml/text.rb#L155-L172

test/test_text_check.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: false
+
+module REXMLTests
+  class TextCheckTester < Test::Unit::TestCase
+    include REXML
+
+    def test_valid_pattern
+      chars = ['&A;', '&:;', '&_;', '&:A;', '&AA.bb-00;']
+      chars.each do |token|
+        text = Text.new(token, false, nil, true)
+        assert_equal(token, text.to_s)
+      end
+
+      character_entities = ['&amp;', '&lt;', '&gt;', '&quot;', '&apos;', '&nbsp;', '&Psi;']
+      character_entities.each do |token|
+        text = Text.new(token, false, nil, true)
+        assert_equal(token, text.to_s)
+      end
+
+      numeric_entities = ['&#13;', '&#34;', '&#9830;', '&#0000000013;', '&#0000000162;', '&#x10FFFF;', '&#1114111;']
+      numeric_entities.each do |token|
+        text = Text.new(token, false, nil, true)
+        assert_equal(token, text.to_s)
+      end
+
+      unicode_entities = ['&#x9;', '&#xa;', '&#xD;', '&#x84;', '&#x9F;', '&#xFDEF;', '&#x10FFFF;', '&#x000000007f;']
+      unicode_entities.each do |token|
+        text = Text.new(token, false, nil, true)
+        assert_equal(token, text.to_s)
+      end
+
+      unicodes = ["&\u00C0;", "&\uFDF0;", "&\u{10000};", "&\u00D6\u0300\u0300;"]
+      unicodes.each do |unicode|
+        text = Text.new(unicode, false, nil, true)
+        assert_equal(unicode, text.to_s)
+      end
+    end
+
+    def test_invalid_pattern
+      chars = ['<', '<;', '&amp', '&42;','&#A;', '&#8;', '&#xB;', '&#x1f;', '&#xD800;', '&#xFFFE;', '&#x110000;', '&#1114112;']
+      chars.each do |token|
+        assert_raise(RuntimeError) { Text.new(token, false, nil, true) }
+      end
+    end
+  end
+end