tests_suite_ssl: remove certificate loading for PSK based tests using `move_handshake_to_state` test function

Suggested enhancement

Allow passing MBEDTLS_PK_NONE in move_handshake_to_state related tests aiming to use PSK key exchange so that no certificate is loaded and test dependencies can be further simplified.

This improvement was proposed by this comment.

Justification

Mbed TLS needs this because it allows to test PSK scenarios with more realistic dependencies.