mb: Don't overwrite any test cmdline-prefix in CrOS HW tests.

HW tests aren't picking up the `luci-auth context ...` prefix. This
should fix that.

Bug: 1057152
Change-Id: I739df6433d1e5979863d6c382882dc6370bf7ae6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252564Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780115}

tools/mb/mb.py

@@ -1509,7 +1509,7 @@ class MetaBuildWrapper(object):
       # If we're testing a CrOS simplechrome build, assume we need to prepare a
       # DUT for testing. So prepend the command to run with the test wrapper.
       if is_cros_device:
-        cmdline = [
+        cmdline += [
             os.path.join('bin', 'cros_test_wrapper'),
             '--logs-dir=${ISOLATED_OUTDIR}',
         ]