Remove usage of testify in persistent_auth_test.go. (#1238)

## What changes are proposed in this pull request?

This PR removes usage of `testify` in `persistent_auth_test`. The change
is not focused on improving the test ergonomy but rather in cutting the
dependency on `testify` in preparation to another PR.

## How is this tested?

N/A

NO_CHANGELOG=true

Author: renaudhartert-db