Fix wttest to not download S3 objects when test is skipped

WT-8927 added a helper function to download objects uploaded to the S3 bucket when an S3 test fails or preserve mode is turned on. There is a bug as part of the logic in the test teardown where the test attempts to download the objects when preserve mode is turned on but the test is skipped. Change this logic such that the test does not try to download the objects if the test is skipped.

Suggested fix:

        if hasattr(self, 'ss_name') and self.ss_name == 's3_store' and not self.skipped and \

            (not passed or WiredTigerTestCase._preserveFiles):

                self.download_objects(self.bucket, self.bucket_prefix)

                self.pr('downloading s3 files')