tarsum: start a test for TarSum

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: adding the layer for "scratch" image

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: test gofmt

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: adding benchmarks

to cover a couple of use-cases:
* 1mb file, using no compression
* 1mb file, using compression
* 1024 1k files, using no compression
* 1024 1k files, using compression

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #5780 from vbatts/vbatts-start_tarsum_test

tarsum: start a test for TarSum