Anchor DateTimeTest to fixed DateTime instead of a variable value based on Time.now#advance#to_datetime. Works around issue on 64-bit platforms with Ruby's Time#to_datetime respecting fractional seconds, and database adapters not respecting them for DateTimes, throwing off before-and-after-save equality test. References #10080, #10073

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@8326 5ecf4fe2-1ee6-0310-87b1-e25e094e27de