[SalesForce] Too many code statements: 200001 … why do this error occur

Don't ever do DML or SOQL within an iteration scope. You can directly perform these on a collection object.

List<Case> lstToDelete = new List<Case>(); //COLLECTION
for (Case obj : [select Id from Case]) {
        //any possible business logic

        //collect objects you wish to delete/update/anything
        lstToDelete.add( obj); //COLLECT

        //never do another soql query for related records in an interation either, collect the id's in a collection object, and then query  as SELECT  fields FROM obj WHERE field IN:collection
    }

delete lstToDelete;  //BULK EXECUTE

In all honesty, if this is your testcode, I'm a bit worried about your production code. Topics as bulkifying code and governer limits are essential to know when coding on the force.com platform.

Creating really big .csv files is a challenge without leveraging the async framework. Try to reduce the calls to that formatCSVString(), or perhaps refactor is such that you only need to call it once per line, rather than one per field.

I second Sdry's point, you may have to break this up into smaller chunks and only process, say, 2000 records at a time. Alternatively, going to the async framework, either through a @future method or a batch job, will get you a limit of 1,000,000 code statements per execution context.

After you construct this .csv string, what are you going to do with it? Can you explain the full use case, end to end, in more detail? What starts off this process and what is the end result?