Convert from gotest.tools/v3/assert -> testify

[cpuguy83]

We have a mix of assertion libraries in the repo.
It'd be nice to consolidate on one.
People seem to be most familiar with testify, so I think that's a good one to go to... although I think most tests are using gotest.tools, so this is more work.

[Harsh-Daga]

@cpuguy83 I can take this up. I’ll work on consolidating the assertion libraries by standardizing on testify, and will migrate existing tests from gotest.tools incrementally to testify

[invidian]

Right before this issue was created I also opened #914, which removes testify. But I'm fine either way (if we have to use assertion library ;) ).

[cpuguy83]

Merged #914 so we at least have things uniform.
Not against using testify still, though.