[autotest] Fix syntax error in tradefed test

Missing brackets around a list of params for string format of an
error.

BUG=chromium:896408
TEST=Manual testing

Change-Id: I96a36a54b90c6ef5c8fb5d3d5001fd6a3d86bbf7
Reviewed-on: https://chromium-review.googlesource.com/c/1287112
Commit-Queue: Keith Haddow <haddowk@chromium.org>
Tested-by: Keith Haddow <haddowk@chromium.org>
Trybot-Ready: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
diff --git a/server/cros/tradefed_test.py b/server/cros/tradefed_test.py
index 7194a23..674f9b7 100644
--- a/server/cros/tradefed_test.py
+++ b/server/cros/tradefed_test.py
@@ -932,7 +932,7 @@
         except (shutil.Error, OSError, IOError) as e:
             raise error.TestFail(
                 'Error: failed to copy test subplan %s to CTS bundle. %s' %
-                test_subplan_file, e)
+                (test_subplan_file, e))
 
     def _should_skip_test(self, _bundle):
         """Some tests are expected to fail and are skipped.