Don't assert fractional seconds can be applied on unsupported adapters

This was passing prior to 20b177b7,
because we were not properly applying our contract that
`model.attr == model.tap(&:save).reload.attr` for this case. Now that
that has been resolved, this test is invalid on some adapters

activerecord/test/cases/dirty_test.rb

@@ -592,6 +592,7 @@ def test_field_named_field
   end
 
   def test_datetime_attribute_can_be_updated_with_fractional_seconds
+    skip "Fractional seconds are not supported" unless subsecond_precision_supported?
     in_time_zone 'Paris' do
       target = Class.new(ActiveRecord::Base)
       target.table_name = 'topics'