Add oidctest package

[ericchiang]

Currently it's pretty hard to write a unit test for this package. Consider adding a wrapper around https://pkg.go.dev/net/http/httptest that spins up a minimal server.

package oidctest

type Server struct {
    PublicKeys []crypto.PublicKey
    SigningKey crypto.PrivateKey
    // ... other configuration
}

type Claims struct {
    // ... fields
}

func (s *Server) Start()
func (s *Server) Close()
func (s *Server) NewIDToken(claims *Claims) string
func (s *Server) NewProvider() *oidc.Provider

[jimlambrt]

hashicorp's cap provides a TestProvider which could possibly be used to write unit tests or you could be "inspired" by it's implementation for something similar here.