scons: Make sure GTests have the right environment variables

SCons currently scrubs the environment variables used by GTests too
aggressively. This breaks systems where libraries are installed in
non-standard locations that need to be specified in
LD_LIBRARY_PATH. Run said tests in the gtest_env SCons environment
which white-lists the important environment variables.

Change-Id: I5fc8fb5e51f09644dc976ee97b21c78ab349bf7d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/6623
Reviewed-by: Gabe Black <gabeblack@google.com>

src/SConscript

@@ -1102,7 +1102,7 @@ def makeEnv(env, label, objsfx, strip=False, disable_partial=False, **kwargs):
             test.dir.File('%s.%s' % (test.target, label)), test_objs))
 
     gtest_target = Dir(new_env['BUILDDIR']).File('unittests.%s' % label)
-    AlwaysBuild(Command(gtest_target, gtests, gtests))
+    AlwaysBuild(gtest_env.Command(gtest_target, gtests, gtests))
 
     progname = exename
     if strip: