raft: use testify package in unit tests

[pav-kv]

What would you like to be added?

Use github.com/stretchr/testify/require and github.com/stretchr/testify/assert packages in all unit tests.

Demo PR: #14708

Why is this needed?

This removes much boilerplate from the tests, and improves speed and quality of understanding and modifying them.

[falser101]

/assign

[serathius]

@falser101 I think the issue is already worked on. #14708
Assigning pavelkalinnikov who send the PR to avoid further confusion.

[serathius]

Closed by #14708

[pav-kv]

This is not done, there will be a few commits making more tests to take advantage of this package, notably raft_test.go. The first PR linked in the description was intentionally small to demo things, but I can make the next PR complete this work.