Add some test cases for `Statement' and `Insert'.

Hi, all.

I have tried writing some test cases for `org.mybatis.scala.mapping.Statement` and `org.mybatis.scala.mapping.Insert`.

I think mybatis-scala becomes much better than now if this library becomes to have appropriate test cases, so I would like to add tests to other sources in mybatis-scala not included in this pull request.

Then, I have a question that we should use which testing framework and testing style. Currently I'm using scalatest because it exists in the `pom.xml` already.

Best regards.