test_date_arith.rb: needs `test_` prefix

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62684 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/date/test_date_arith.rb

@@ -4,7 +4,7 @@ require 'date'
 
 class TestDateArith < Test::Unit::TestCase
 
-  def new_offset
+  def test_new_offset
     d = DateTime.new(2002, 3, 14)
     assert_equal(Rational(9, 24), d.new_offset(Rational(9, 24)).offset)
     assert_equal(Rational(9, 24), d.new_offset('+0900').offset)