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"

tests/data_files/dir4/Readme

@@ -40,3 +40,8 @@ cert71.crt (max_pathlen=1) -> cert72.crt -> cert73.crt (self signed) -> cert74.c
 ```
 cert81.crt -> cert82.crt (max_pathlen=0) -> cert83.crt
 ```
+
+9. zero pathlen constraint on trusted root (valid)
+```
+cert91.crt (max_pathlen=0) -> cert92.crt
+```

tests/data_files/dir4/cert91.crt

@@ -0,0 +1,30 @@
+-----BEGIN CERTIFICATE-----
+MIIFMDCCAxigAwIBAgIBWzANBgkqhkiG9w0BAQsFADAxMQ8wDQYDVQQDEwZSb290
+IDkxETAPBgNVBAoTCG1iZWQgVExTMQswCQYDVQQGEwJVSzAeFw0wMTAxMDEwMDAw
+MDBaFw0zMDEyMzEyMzU5NTlaMDExDzANBgNVBAMTBlJvb3QgOTERMA8GA1UEChMI
+bWJlZCBUTFMxCzAJBgNVBAYTAlVLMIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIIC
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+OLhnrgR/CdJLip9UYegD7+7ft1yW4MCvlmtm96adg/aaLMHQ+9fSSFpSTwg8N49d
+zH7qBB002FF0QFYLXW8WMfGSSTuu/c5uQVWWPkVlQMFyAHfIv+/2OfXPERCJw/MQ
+VyidHIQ0VcUWVS981uEWZmBq3LKB3BTrHURo/nXeFhD5F1H4XUWyzY1jzemL+noE
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+Q4orMRiPimzeWohzS2X9WIbWVOodocPYl1qxK9uOHeYmuujapDVeGLYjcQbZuknO
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+HwYDVR0jBBgwFoAU6Emig07wVTtIf18xo/EByrTqBIgwDQYJKoZIhvcNAQELBQAD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+-----END CERTIFICATE-----

tests/data_files/dir4/cert92.crt

@@ -0,0 +1,30 @@
+-----BEGIN CERTIFICATE-----
+MIIFKTCCAxGgAwIBAgIBXDANBgkqhkiG9w0BAQsFADAxMQ8wDQYDVQQDEwZSb290
+IDkxETAPBgNVBAoTCG1iZWQgVExTMQswCQYDVQQGEwJVSzAeFw0wMTAxMDEwMDAw
+MDBaFw0zMDEyMzEyMzU5NTlaMDAxDjAMBgNVBAMTBUVFIDkyMREwDwYDVQQKEwht
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+FBCJhVAwZb2y95tawcV6YRLphOhDh1YgaacqfTHR/vQpe97ZP7cDd4SvoLSixO6X
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+3NL2cjO2LdqJfNbI/l5OxSDD0JJAKBJsZY6J1JFf+HA8om4mxWgoWRn/Cp+ySvuZ
+zVIEwJX+hvG8N7DK3tBA1Td0ZRsYK048yhd3EYVt84swHfS1EB5K+eGiP/sP/BH6
+NVJPttTB1a4y8e3SjG2y/WAoAevXV0/62SIy1vjt/sKnhOb0NcOhXXhkrB5tuqVA
+At3K9sR+iGvuwbHwjAvvvMvnWSSAbTQBwHYCqvGflLGui+pu7lKX7TtdyZXKthGJ
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+jJACWlRRKE7eLxTKcwmK4Lr0aEpol5D+r4IVLWIbTwKMBs/bB7oXlqN+dHKCiRD/
+MKCLG7+G/rg/fdX8KT4idpjM75S/YfzEd6xFR+bguswZzB/UHR9nweq4FDD44dXr
+z9TiWxce+2rw/pnqEX9yDuIE+/p4mUYy13k4JCeIvp1zFS9iJGNiqjiYnc3Xs0o0
+SxgyAkLxVBJ1Kz1Wosjq4zx/ZRfIm7L+59kWRfqNhV83dnu/NDDNd1N6deS1E9Jm
+bAUU5BudGpd9Q/KPs1Hfs0YZsZC7xlxydAYXgOIVVDjg0PmS9P0ZPejPX3sslgp8
+3i8qWBo1ZdjA/sl4+GK85hgToGWy86aybvShCCc=
+-----END CERTIFICATE-----

tests/suites/test_suite_x509parse.data

@@ -390,10 +390,14 @@ X509 CRT verify chain #8 (self signed maxpathlen root)
 depends_on:POLARSSL_PEM_C:POLARSSL_FS_IO:POLARSSL_SHA2_C
 x509_crt_verify_chain:"data_files/dir4/cert61.crt data_files/dir4/cert63.crt data_files/dir4/cert62.crt":"data_files/dir4/cert61.crt":0
 
-X509 CRT verify chain #9 (self signed maxpathlen root)
+X509 CRT verify chain #9 (zero pathlen first intermediate, valid)
 depends_on:POLARSSL_PEM_C:POLARSSL_FS_IO:POLARSSL_SHA2_C
 x509_crt_verify_chain:"data_files/dir4/cert83.crt data_files/dir4/cert82.crt":"data_files/dir4/cert81.crt":0
 
+X509 CRT verify chain #10 (zero pathlen root, valid)
+depends_on:POLARSSL_PEM_C:POLARSSL_FS_IO:POLARSSL_SHA2_C
+x509_crt_verify_chain:"data_files/dir4/cert92.crt":"data_files/dir4/cert91.crt":0
+
 X509 Parse Selftest
 depends_on:POLARSSL_MD5_C:POLARSSL_PEM_C:POLARSSL_SELF_TEST
 x509_selftest: