Adopt stretchr/testify

Migrate hand-written test assertions to the github.com/stretchr/testify library and add the dependency to go.mod. Converts error guards to require/assert NoError/Error, length checks to Len, equality checks to Equal/NotEqual, nil checks to Nil/NotNil, and boolean checks to True/False, then adds the testify require. Does not sync go.sum; a go mod tidy is still needed to complete resolution.