chromeos: Move system logs from host-side into a subdir.

Same thing that crrev.com/c/2219679 did, but for tests invoked via
host-side cmd (eg: telemetry) rather than device-side cmd (eg: gtests).

Bug: 1087407
Change-Id: I514a01583bbfea6a4a600d738587d8663ae0adec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222901Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773360}

build/chromeos/test_runner.py

@@ -724,7 +724,10 @@ def host_cmd(args, unknown_args):
   if args.logs_dir:
     for log in SYSTEM_LOG_LOCATIONS:
       cros_run_test_cmd += ['--results-src', log]
-    cros_run_test_cmd += ['--results-dest-dir', args.logs_dir]
+    cros_run_test_cmd += [
+        '--results-dest-dir',
+        os.path.join(args.logs_dir, 'system_logs')
+    ]
 
   test_env = setup_env()
   if args.deploy_chrome: