fix: misc fixes in pub/gpg/tests (#38809)

- The pub registry reported the oldest version as `latest`, because the
descriptor slice is sorted ascending but the first element was used.
- Verifying a GPG or SSH key flashed success and redirected after
already writing an error response, so a failure was reported as a
success with an empty key id.
- Test packages sharing redis could tear down each other's server.
`PrepareTestRedis` started its own on the well-known port, so a package
running in parallel borrowed it and lost it when the owner's cleanup
fired. It now listens on a socket of its own.

routers/web/user/setting/keys.go

@@ -161,6 +161,7 @@ func KeysPost(ctx *context.Context) {
 			default:
 				ctx.ServerError("VerifyGPG", err)
 			}
+			return
 		}
 		ctx.Flash.Success(ctx.Tr("settings.verify_gpg_key_success", keyID))
 		ctx.Redirect(setting.AppSubURL + "/user/settings/keys")
@@ -232,6 +233,7 @@ func KeysPost(ctx *context.Context) {
 			default:
 				ctx.ServerError("VerifySSH", err)
 			}
+			return
 		}
 		ctx.Flash.Success(ctx.Tr("settings.verify_ssh_key_success", fingerprint))
 		ctx.Redirect(setting.AppSubURL + "/user/settings/keys")