Re-enable TestSpec

[KyleMaas]

Doesn't actually appear to be broken, as long as you have envelope-spec downloaded like the test says. Hoping the CI system knows how to do that.

Fixes #275

See also #237

[KyleMaas]

Apparently the CI system does not know how to download that, which is probably why it was failing.

[KyleMaas]

A different test failed in that last CI run. Looks like this one worked. The failing test has been noted as #277, since that doesn't appear to be consistent.

So...what I did here was integrate the git clone command into the Makefile for make test. The reasons I did this rather than use a git submodule, and then clean it up after the test suite is done running, are:

1. Submodules are a pain.
2. This is only necessary for the one specific test. It's not relevant to building binaries or running anything other than the test suite.
3. If the test is individually run, it tells the user what they need to do. So the only use case we really need to solve is the CI suite.

So, for those reasons, I feel that adding this into the Makefile is the most appropriate place for this.

[KyleMaas]

@decentral1se

This one should be ready for review when you have a chance. The test failure for the last commit was a different test than this one, and this one passed:

https://github.com/ssbc/go-ssb/actions/runs/3762663556/jobs/6395559642#step:9:196

[decentral1se]

LGTM 🚀

[KyleMaas]

Thanks!