* terraform test: add variable definitions to test files
Currently, `terraform test` attempts to work out the type of any external variables by delaying evaluation until each run block executes so it can use the definitions within the run blocks's module. This means that the values of variables can technically change between run blocks which isn't ideal.
This commit is the first in a chain which will move the evaluation of variables into the terraform test graph. We need to give the users the option of specifying the type for external variables within the file as these variables are now going to be assessed outside of the context of a run block. To do this, we introduce the optional variable blocks that are added by this commit.
* update comment
