Improve test coverage

[vpavic]

As evident by some recent issues like #32 and #37, test coverage in this project is quite low.

For example, SignedDataVerifier only verifies test notification type and not others that are truly relevant and ReceiptUtility is completely untested.

[alexanderjordanbaker]

Hello @vpavic we have now increased our test coverage and aim to test all major flows in some recent PRs to add additional coverage.

[alexanderjordanbaker]

This was resolved in #51

[vpavic]

Thanks for the update @alexanderjordanbaker, I just went through all the changes that were merged over the past few weeks and the situation is now indeed better.

I'd just suggest to add some more test cases for server notifications, because the only tested notification is one of type TEST and those don't really have much data on them and therefore do not adequately test the whole decoding and verification functionality.

[alexanderjordanbaker]

@vpavic I think this is the test you are looking for, it does a more comprehensive test of the actual decoding of notifications https://github.com/apple/app-store-server-library-java/blob/main/src/test/java/com/apple/itunes/storekit/model/ResponseBodyV2DecodedPayloadTest.java

The tests treat the verification pieces (x5c header that kind of thing) separately from the making sure the model decoding is correct

[vpavic]

Thanks, I missed that one, it indeed is what I was looking for.