implement delete manifest tests

[cpanato]

I was learning this repo to try to use in one personal thing and saw that some tests were pending to add.

This PR tries to implement the tests for crane.Delete, I'm not 100% if it is correct but it is a tryout 😄

if sounds reasonable I will try to implement the other missing tests

/cc @jonjohnsonjr

[imjasonh]

lgtm in general, just a small style/consistency nit.

Thanks for this contribution! 🎉

[codecov-io]

Codecov Report

Merging #915 (fc5c445) into master (38d292e) will decrease coverage by 0.26%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #915      +/-   ##
==========================================
- Coverage   72.67%   72.41%   -0.27%     
==========================================
  Files         108      108              
  Lines        4696     4713      +17     
==========================================
  Hits         3413     3413              
- Misses        773      789      +16     
- Partials      510      511       +1     

Impacted Files	Coverage Δ
pkg/registry/manifest.go	72.11% <0.00%> (-14.10%)	⬇️

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[jonjohnsonjr]

Couple nits, otherwise lgtm.

[jonjohnsonjr]

if sounds reasonable I will try to implement the other missing tests

That would be wonderful, if you have the time! Thanks :)

[jonjohnsonjr]

Thanks!