[bigdecimal] Fix deprecation warning test

test/bigdecimal/test_bigdecimal.rb

@@ -611,9 +611,13 @@ class TestBigDecimal < Test::Unit::TestCase
   end
 
   def test_precs_deprecated
+    saved = Warning[:deprecated]
+    Warning[:deprecated] = true
     assert_warn(/BigDecimal#precs is deprecated and will be removed in the future/) do
       BigDecimal("1").precs
     end
+  ensure
+    Warning[:deprecated] = saved
   end
 
   def test_precs