Only exercise Brainpool curve keys on one algorithm
Only exercise Brainpool curve keys on one algorithm

There's nothing wrong with ECC keys on Brainpool curves,
but operations with them are very slow. So we only exercise them
with a single algorithm, not with all possible hashes. We do
exercise other curves with all algorithms so test coverage is
perfectly adequate like this.

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls