[ruby/bigdecimal] Add truffleruby in CI

https://github.com/ruby/bigdecimal/commit/5a25e26e08

test/bigdecimal/test_bigdecimal.rb

@@ -294,7 +294,11 @@ class TestBigDecimal < Test::Unit::TestCase
   end
 
   def test_s_allocate
-    assert_raise_with_message(TypeError, /allocator undefined for BigDecimal/) { BigDecimal.allocate }
+    if RUBY_ENGINE == "truffleruby"
+      assert_raise_with_message(NoMethodError, /undefined.+allocate.+for BigDecimal/) { BigDecimal.allocate }
+    else
+      assert_raise_with_message(TypeError, /allocator undefined for BigDecimal/) { BigDecimal.allocate }
+    end
   end
 
   def test_s_new