tests: Fix the stats unit test.

tests: Fix the stats unit test.

This has been broken since February. The interface for opening
initializing where the stats output should go was changed, but the
test wasn't updated.

Change-Id: I54bd8be15bf870352d5fcfad95ded28d87c7cc5a
Reviewed-on: https://gem5-review.googlesource.com/6001
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>