This is intended as a replacement for sharness. These are vanilla Go
tests which can be run in your IDE for quick iteration on end-to-end
CLI tests.
This also removes IPTB by duplicating its functionality in the test
harness. This isn't a big deal...IPTB's complexity is mostly around
the fact that its state needs to be saved to disk in between `iptb`
command invocations, and that it uses Go plugins to inject
functionality, neither of which are relevant here.
If we merge this, we'll have to live with bifurcated tests for a while
until they are all migrated. I'd recommend we self-enforce a rule
that, if we need to touch a sharness test, we migrate it and one more
test over to Go tests first. Then eventually we will have migrated
everything.
