Slim TableTable: manage variable assignment in VariableStore

Variable set ($X=) was not possible on a TableTable.
This pull request propose a simple solution to manage them.

Note that I don't understand why there is two different stores (without any link between them) for symbols values: SlimTestContext.symbols and SlimExecutionContext.variables

Yes you get it :) And now I have understood the difference between the two packages. Eventually, this solution is not simple as I thought.
I've also tested other solutions but none of them is clean. Have you got another idea to manage this feature differently?