Merge remote-tracking branch 'hanno/iotssl-1341-optional-certificate-verification-needs-ca-chain_backport-1.3' into mbedtls-1.3

* hanno/iotssl-1341-optional-certificate-verification-needs-ca-chain_backport-1.3:
  Add tests for missing CA chains and bad curves.
  Fix implementation of VERIFY_OPTIONAL verification mode