tests: Use --silent-redirect when running gem5 tests.

The redirects are to temp files which are going to be deleted at the end
of the test, and there's no reason to clutter up the output with nonsense
paths to non-existent files.

Change-Id: Id312b1f25f03d7c57318ca0c58902e4193bd1e91
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50589
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/suite.py

@@ -168,9 +168,10 @@ def _create_test_run_gem5(config, config_args, gem5_args):
         gem5 = fixtures[constants.gem5_binary_fixture_name].path
         command = [
             gem5,
-            '-d',  # Set redirect dir to tempdir.
+            '-d', # Set redirect dir to tempdir.
             tempdir,
-            '-re',# TODO: Change to const. Redirect stdout and stderr
+            '-re', # TODO: Change to const. Redirect stdout and stderr
+            '--silent-redirect',
         ]
         command.extend(_gem5_args)
         command.append(config)