Update to testify v1.2.2

What does this MR do?

Update to https://github.com/stretchr/testify to the latest version (v1.2.1)

Why was this MR needed?

• We have an outdated vendored version
• The Maybe method that was introduced is quite useful as it might lead into less flaky tests. For example !1058 (merged) we do a check on Close but we don't really care if it get's called or not since it's not the main purpose of those tests and those tests might finish earlier before the Close method is closed since it is done concurrently.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• [-] Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #3843 (closed)