scons: Build GTests in the directory they're declared.

scons: Build GTests in the directory they're declared.

This will let us move unit tests into the directory of the code they're
testing.

Change-Id: I611582eb00a06a848f3ce7da89f64e42660cb844
Reviewed-on: https://gem5-review.googlesource.com/6141
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>