Friction for tinygo users

[dkegel-fastly]

tinygo as of release 0.22 does not support goleak; this isn't particularly surprising, as tinygo still only provides stubs for several things goleak relies upon.

A number of apps - particularly, the one I'm trying to ship :-) - would pass tests under both go and tinygo if goleak sensed the tinygo build constraint and provided a minimal stub while tinygo matures.

This would be an elegant way to reduce friction for goleak users who must pass tests with both go and tinygo.

I'll provide a pull request with the simple change required. I think this is sufficient; works for my app, at least.