Let’s Encrypt provides a separate staging environment for testing. It is fully-functional, the issued certificates are just signed by a bogus fake CA.
To use the staging environment with LiveConfig, you need to run the following SQL statement with the LiveConfig database:
INSERT INTO SSLPROVIDERS (SP_NAME, SP_LOGO, SP_MODULE, SP_DESCRIPTION, SP_DATA) VALUES ("Let's Encrypt Staging", "logo-letsencrypt.svg", "ssl-letsencrypt", "Staging environment of Let's Encrypt - only for testing purposes.", "https://acme-staging-v02.api.letsencrypt.org/directory");
In LiveConfig, you then find this new SSL provider at “Hosting” -> “SSL Certificates” -> “SSL Providers”.