Create mock infrastructure for implementation testing

[magik6k]

It would be amazing to be able to have a test utility cobbling blockstore/resolver/... together for the purpose of testing ipld modules.

For git a test would add a git repo objects to that and test all the things that are currently rather hard to do, like path traversal / resolving.

[Stebalien]

I tend to write interface tests like this by:

• Adding a test package.
• Exposing a single SubtestAll(t *testing.T) function.
• Have all implementations test against it.