PK tests: use PSA to generate keypairs when USE_PSA is enabled

[mpg]

Context: see #6839; this is a step towards ECPa.PK.

In test_suite_pk.function, we use ECP functions to generate ECC key pairs in two places:

• mbedtls_ecp_gen_keypair() in pk_genkey();
• mbedtls_ecp_gen_key() in pk_psa_sign().
  This is a dependency on ECP arithmetic, which we're trying to avoid.

Instead, when MBEDTLS_USE_PSA_CRYPTO is enabled, we can use an approach based on psa_generate_key(). This task is to implement it.

Notes:

• pk_psa_sign() already depends on MBEDTLS_USE_PSA_CRYPTO.
• We don't want duplicated call, anything that can be shared between those two places should be (might be partially pre-existing).

Prototype: #7357