Add github-specific tests in callback_handler_github_test.go

Co-authored-by: Ryan Richard <richardry@vmware.com>

internal/upstreamgithub/upstreamgithub.go

@@ -5,6 +5,7 @@
 package upstreamgithub
 
 import (
+	"context"
 	"net/http"
 
 	"golang.org/x/oauth2"
@@ -87,6 +88,11 @@ func (p *Provider) GetAuthorizationURL() string {
 	return p.c.OAuth2Config.Endpoint.AuthURL
 }
 
+func (p *Provider) ExchangeAuthcode(_ context.Context, _ string, _ string) (string, error) {
+	//TODO implement me
+	panic("implement me")
+}
+
 // GetConfig returns the config. This is not part of the interface and is mostly just for testing.
 func (p *Provider) GetConfig() ProviderConfig {
 	return p.c