scons: Fix the regression tests.

scons: Fix the regression tests.

A recent scons cleanup introduced a small bug when setting the variant path of
the regression test SConscript. When turned into a relative path, the
directory can be interpreted differently depending on what it's considered
relative to. When left as a scons node, there is no ambiguity.

Change-Id: I162afdc4fcef81db096ec8304f81bc98fa6bf899
Reviewed-on: https://gem5-review.googlesource.com/5221
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>