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>

src/SConscript

@@ -1117,7 +1117,7 @@ def makeEnv(env, label, objsfx, strip=False, disable_partial=False, **kwargs):
 
     # Set up regression tests.
     SConscript(os.path.join(env.root.abspath, 'tests', 'SConscript'),
-               variant_dir=Dir('tests').Dir(new_env.Label).path,
+               variant_dir=Dir('tests').Dir(new_env.Label),
                exports={ 'env' : new_env }, duplicate=False)
 
 # Start out with the compiler flags common to all compilers,