fix discovery teardown

[waynr]

Discovery test teardown was broken prior to this because it would attempt to delete a blob before deleting at least five manifests left over from the setup that still reference it. Most registries probably don't care about deleting content that's referenced by other content, which is probably why this works for others.

Also, I was seeing GET /v2/<name>/tags/list?n=0 requests and then out of bounds indexing into the resulting slice because of the weird logic used to calculate what the n parameter should be in the tag listing endpoint tests.

Fixes #457.

[mikebrow]

pls sign commits

[sudo-bmitch]

@waynr do you have time to work on the changes requested? If not, I can take a stab at it.

[waynr]

@sudo-bmitch I accepted your suggested changes, thanks for the reminder!