The SSL test programs can now use the PSA RNG, and the PSA RNG can use
an external RNG. The build conditions hadn't been updated and didn't
cover the case when MBEDTLS_TEST_USE_PSA_CRYPTO_RNG is enabled but
MBEDTLS_USE_PSA_CRYPTO is disabled. Fix this.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>