HandleCreateCredentialSuccessfully creates an HTTP handler at `/credentials` on the test handler mux that tests credential creation.

HandleDeleteCredentialSuccessfully creates an HTTP handler at `/credentials` on the test handler mux that tests credential deletion.

HandleGetCredentialSuccessfully creates an HTTP handler at `/credentials` on the test handler mux that responds with a single credential.

HandleListCredentialsSuccessfully creates an HTTP handler at `/credentials` on the test handler mux that responds with a list of two credentials.

HandleUpdateCredentialsSuccessfully creates an HTTP handler at `/credentials` on the test handler mux that tests credentials update.

CreateRequest provides the input to a Create request.

GetOutput provides a Get result.

ListOutput provides a single page of Credential results.

UpdateOutput provides an update result.

UpdateRequest provides the input to as Update request.

ExpectedCredentialsSlice is the slice of credentials expected to be returned from ListOutput.

SecondCredentialUpdated is how SecondCredential should look after an Update.