Add tests for REXML::Text.check (#165)

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 +1,92 @@
+# frozen_string_literal: false
+
+module REXMLTests
+  class TextCheckTester < Test::Unit::TestCase
+
+    def check(string)
+      REXML::Text.check(string, REXML::Text::NEEDS_A_SECOND_CHECK, nil)
+    end
+
+    def assert_check(string)
+      assert_nothing_raised { check(string) }
+    end
+
+    def assert_check_failed(string, illegal_part)
+      message = "Illegal character #{illegal_part.inspect} in raw string #{string.inspect}"
+      assert_raise(RuntimeError.new(message)) do
+        check(string)
+      end
+    end
+
+    class TestValid < self
+      def test_entity_name_start_char_colon
+        assert_check('&:;')
+      end
+
+      def test_entity_name_start_char_under_score
+        assert_check('&_;')
+      end
+
+      def test_entity_name_mix
+        assert_check('&A.b-0123;')
+      end
+
+      def test_character_reference_decimal
+        assert_check('&#0162;')
+      end
+
+      def test_character_reference_hex
+        assert_check('&#x10FFFF;')
+      end
+
+      def test_entity_name_non_ascii
+        # U+3042 HIRAGANA LETTER A
+        # U+3044 HIRAGANA LETTER I
+        assert_check("&\u3042\u3044;")
+      end
+
+      def test_normal_string
+        assert_check("foo")
+      end
+    end
+
+    class TestInvalid < self
+      def test_lt
+        assert_check_failed('<;', '<')
+      end
+
+      def test_lt_mix
+        assert_check_failed('ab<cd', '<')
+      end
+
+      def test_entity_reference_missing_colon
+        assert_check_failed('&amp', '&')
+      end
+
+      def test_character_reference_decimal_invalid_value
+        # U+0008 BACKSPACE
+        assert_check_failed('&#8;', '&#8;')
+      end
+
+      def test_character_reference_format_hex_0x
+        # U+0041 LATIN CAPITAL LETTER A
+        assert_check_failed('&#0x41;', '&#0x41;')
+      end
+
+      def test_character_reference_format_hex_00x
+        # U+0041 LATIN CAPITAL LETTER A
+        assert_check_failed('&#00x41;', '&#00x41;')
+      end
+
+      def test_character_reference_hex_surrogate_block
+        # U+0D800 SURROGATE PAIR
+        assert_check_failed('&#xD800;', '&#xD800;')
+      end
+
+      def test_entity_name_non_ascii_symbol
+        # U+00BF INVERTED QUESTION MARK
+        assert_check_failed('&\u00BF;', '&')
+      end
+    end
+  end
+end