Simplify Vault SPI tests by using client secret references

Once [keycloak#39650](https://www.github.com/keycloak/keycloak/pull/39650) is merged, update Vault SPI [tests](https://github.com/Nordix/keycloak-secrets-vault-provider/blob/main/src/test/java/io/github/nordix/keycloak/services/vault/SecretsProviderIT.java) to use a client secret instead of testing with identity brokering which depend on Chromium and Selenium. 