Add test case for root with max_pathlen=0
This was already working but not tested so far
(Test case from previous commit still failing.)
Test certificates generated with:
programs/pkey/gen_key type=ec ec_curve=secp256r1 filename=cert91.key
programs/pkey/gen_key type=ec ec_curve=secp256r1 filename=cert92.key
programs/x509/cert_write serial=91 output_file=cert91.crt is_ca=1 \
issuer_key=cert91.key issuer_name="CN=Root 9,O=mbed TLS,C=UK" \
selfsign=1 max_pathlen=0
programs/x509/cert_write serial=92 output_file=cert92.crt \
issuer_key=cert91.key issuer_name="CN=Root 9,O=mbed TLS,C=UK" \
subject_key=cert92.key subject_name="CN=EE 92,O=mbed TLS,C=UK"
mv cert9?.crt tests/data_files/dir4
rm cert9?.key
