Fix TarSum iteration test

I noticed that 3 of the tarsum test cases had expected a tarsum with
a sha256 hash of
  e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
As I've been working with sha256 quite a bit lately, it struck me that
this is the initial digest value for sha256, which means that no data
was processed. However, these tests *do* process data. It turns out that
there was a bug in the test handling code which did not wait for tarsum
to end completely. This patch corrects these test cases.

I'm unaware of anywhere else in the code base where this would be an issue,
though we definitily need to look out in the future to ensure we are
completing tarsum reads (waiting for EOF).

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #9463 from jlhawn/tarsum_test_fix

Fix TarSum iteration test