tests: Use relative path for python3 compliance

Change-Id: Ie18c52982e2083d0fc2723147f2493b39bcb3786
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26743
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/testing/tests.py

@@ -38,7 +38,7 @@
 from abc import ABCMeta, abstractmethod
 import os
 from collections import namedtuple
-from units import *
+from .units import *
 from .helpers import FileIgnoreList
 from .results import TestResult
 import shutil

tests/testing/units.py

@@ -45,8 +45,8 @@ import subprocess
 import sys
 import traceback
 
-from results import UnitResult
-from helpers import *
+from .results import UnitResult
+from .helpers import *
 
 _test_base = os.path.join(os.path.dirname(__file__), "..")