scons: Merge reading test SConscripts into makeEnv.

scons: Merge reading test SConscripts into makeEnv.

We're already visiting each of the environments, there's no reason to
track them all and then set up test SConscripts on a second pass.

Change-Id: I2d1166f58ac907e874d6ad5de7bd53ff7ad645f8
Reviewed-on: https://gem5-review.googlesource.com/2980
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>