[Bug #18964] Add test for rb_econv_append

Author: nobu

test/-ext-/econv/test_append.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: false
+require 'test/unit'
+require "-test-/econv"
+
+class Test_EConvAppend < Test::Unit::TestCase
+  def test_econv_str_append_valid
+    ec = Bug::EConv.new("utf-8", "cp932")
+    dst = "\u3044".encode("cp932")
+    ret = ec.append("\u3042"*30, dst)
+    assert_same(dst, ret)
+    assert_not_predicate(dst, :ascii_only?)
+    assert_predicate(dst, :valid_encoding?)
+  end
+
+  def test_econv_str_append_broken
+    ec = Bug::EConv.new("utf-8", "cp932")
+    dst = ""
+    ret = ec.append("\u3042"*30, dst)
+    assert_same(dst, ret)
+    assert_not_predicate(dst, :ascii_only?)
+    assert_not_predicate(dst, :valid_encoding?)
+  end
+end