Remove direct usage of github.com/stretchr/testify in pkg/spire

[vdemeester]

The tektoncd/pipeline project generally relies on the standard library for testing. The github.com/stretchr/testify dependency is only directly used in a few test files within the pkg/spire package.

This issue tracks the work to migrate these tests to use the standard library and remove the direct dependency on github.com/stretchr/testify.