Make SSPI auth mockable (#27036)

Before, the SSPI auth is only complied for Windows, it's difficult to
test and it breaks a lot.

Now, make the SSPI auth mockable and testable.

services/auth/sspiauth_posix.go

@@ -0,0 +1,30 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !windows
+
+package auth
+
+import (
+	"errors"
+	"net/http"
+)
+
+type SSPIUserInfo struct {
+	Username string   // Name of user, usually in the form DOMAIN\User
+	Groups   []string // The global groups the user is a member of
+}
+
+type sspiAuthMock struct{}
+
+func (s sspiAuthMock) AppendAuthenticateHeader(w http.ResponseWriter, data string) {
+}
+
+func (s sspiAuthMock) Authenticate(r *http.Request, w http.ResponseWriter) (userInfo *SSPIUserInfo, outToken string, err error) {
+	return nil, "", errors.New("not implemented")
+}
+
+func sspiAuthInit() error {
+	sspiAuth = &sspiAuthMock{} // TODO: we can mock the SSPI auth in tests
+	return nil
+}