Correct TarSum benchmarks: 9kTar and 9kTarGzip

These two cases did not actually read the same content with each iteration
of the benchmark. After the first read, the buffer was consumed. This patch
corrects this by using a bytes.Reader and seeking to the beginning of the
buffer at the beginning of each iteration.

Unfortunately, this benchmark was not actually as fast as we believed. But
the new results do bring its results closer to those of the other benchmarks.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Correctly close generated benchmark archives

Another update to TarSum tests, this patch fixes an issue where
the benchmarks were generating archives incorrectly by not closing
the tarWriter.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #9479 from jlhawn/tarsum_test_fix

Correct TarSum benchmarks: 9kTar and 9kTarGzip