tests: Remove Python 2.7 glue code

Remove uses of six and from __future__ imports as they are no longer
needed.

Change-Id: I74b5250722abe1e202f31a9ec1d4cc04039df168
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39757
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/main.py

@@ -5,7 +5,6 @@ loaders.
 
 Discovers and runs all tests from a given root directory.
 '''
-from __future__ import print_function
 
 import sys
 import os