Shuffle the logic in mbedtls_test_psa_exported_key_sanity_check()
somewhat. The resulting behavior changes are:
* Always check the exported length against PSA_EXPORT_KEY_OUTPUT_SIZE,
even for unstructured key types.
* Always complain if a key type is not explicitly covered, not just
for public keys.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>