test_import_export: Use get_output_dir() helper.
test_import_export: Use get_output_dir() helper.

That's much briefer and cleaner than repeating the os.path.join
expression and the helper already exists.

zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,zulip/zulip