[llvm-exegesis] Rename range based for loop variable in a unit test so its...

[llvm-exegesis] Rename range based for loop variable in a unit test so its different than the container being iterated over. NFC

It seems like gcc 5.5 wants to iterate over the new variable instead
of the container that lives outside the loop. But of course this
new container is empty.

Plus using a different variable names makes the code more readable.