Don't force a ECC CA when a custom CA is passed with `-A`

The following config would fail `../configure --enable-opensslall CFLAGS="-DOPENSSL_COMPATIBLE_DEFAULTS" && make -j check`. This is because `test-fails.conf` `ECC no signer error` test expects a failure while the ECC CA was being added as a trusted cert due to  `OPENSSL_COMPATIBLE_DEFAULTS`.