test: Add `non_admin_secret_handling` to example configuration

Signed-off-by: David Runge <dvzrv@archlinux.org>

signstar-configure-build/tests/fixtures/example.toml

 iteration = 1
 admin_secret_handling = "shamirs-secret-sharing"
+non_admin_secret_handling = "systemd-creds"
 [[connections]]
 url = "https://localhost:8443/api/v1/"
 tls_security = "Unsafe"