Disable the policy tree exponential growth test conditionally

If there is no EC specified, the test won't pass.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/20572)

(cherry picked from commit f5935fcf)