fix mysql varchar limit

Hi again,

While trying to target mysql, I faced a problem at the table creation. In fact mysql needs a length for varchars.

So I tried to slip a limit into config files and an autolimit for varchars on mysql targets, I added some tests too.

I just draw your attention on the fact that the test are not similar to the previously done tests, and are tweaking around a little, so if you see a better to do this, I'm all ears ;-)

Cheers!