add test cases for tar module

[fulldecent]

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[Jorropo]

Thx a lot, more tests is always nice.

Just so you know we want to discontinue this package, users who want a similar feature would be better served by a content aware unixfs chunker as this code and format (ipld tar) has some pitfalls.

[Jorropo]

Fuck I wanted to enable automerge not merge.
I'm surprised this was disabled.

I'll submit a revert if this breaks CI on master.