Add testify, and update tests to use it rather than expect

[skelouse]

Checklist

• Are you running the latest v3 release? The list of releases is here.
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What problem does this solve?

Simplifies tests, i.e

if err != nil {
	t.Error(err)
	return
}

Becomes

assert.NilError(t, err)

Solution description

import stretchr/testify in tests and replace value checking in tests with testify

[skelouse]

@dearchap was there a reason for the close? I opened this based on a comment on the mentioned PR.

[dearchap]

Ah ok. Sorry I thought that PR was the fix for this.