[test] Be even more verbose when killing hanging tests fails

Fixed problem with to_string

Bug: v8:9855
Change-Id: I5fdd9e77e78a77ab5a9aad833d8ac71bebd1a291
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1886910Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64613}

tools/testrunner/local/command.py

@@ -134,7 +134,7 @@ class BaseCommand(object):
       self._kill_process(process)
     except OSError as e:
       print(e)
-      started_as = self._to_args_list().to_string(relative=True)
+      started_as = self.to_string(relative=True)
       print("Unruly process started as:\n  %s\n" % started_as)
       sys.stdout.flush()
       pass