Add test case for first intermediate max_pathlen=0

!!! This test case is currently failing !!!
(See fix in next-next commit.)

Test certificates generated with the following script:
(using mbed TLS 2.2 as 1.2 does not hove those programs)

programs/pkey/gen_key type=rsa filename=cert81.key
programs/pkey/gen_key type=rsa filename=cert82.key
programs/pkey/gen_key type=rsa filename=cert83.key

programs/x509/cert_write serial=81 output_file=cert81.crt is_ca=1 \
    issuer_key=cert81.key issuer_name="CN=Root 8,O=mbed TLS,C=UK" \
    selfsign=1
programs/x509/cert_write serial=82 output_file=cert82.crt is_ca=1 \
    issuer_key=cert81.key issuer_name="CN=Root 8,O=mbed TLS,C=UK" \
    subject_key=cert82.key subject_name="CN=Int 82,O=mbed TLS,C=UK" \
    max_pathlen=0
programs/x509/cert_write serial=83 output_file=cert83.crt \
    issuer_key=cert82.key issuer_name="CN=Int 82,O=mbed TLS,C=UK" \
    subject_key=cert83.key subject_name="CN=EE 83,O=mbed TLS,C=UK"

tests/data_files/dir4/Readme

@@ -36,3 +36,7 @@ cert61.crt (max_pathlen=1) -> cert62.crt -> cert63.crt
 cert71.crt (max_pathlen=1) -> cert72.crt -> cert73.crt (self signed) -> cert74.crt -> cert74.crt
 ```
 
+8. zero pathlen constraint on first intermediate CA (valid)
+```
+cert81.crt -> cert82.crt (max_pathlen=0) -> cert83.crt
+```

tests/data_files/dir4/cert81.crt

@@ -0,0 +1,30 @@
+-----BEGIN CERTIFICATE-----
+MIIFLTCCAxWgAwIBAgIBUTANBgkqhkiG9w0BAQsFADAxMQ8wDQYDVQQDEwZSb290
+IDgxETAPBgNVBAoTCG1iZWQgVExTMQswCQYDVQQGEwJVSzAeFw0wMTAxMDEwMDAw
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+-----END CERTIFICATE-----

tests/data_files/dir4/cert82.crt

@@ -0,0 +1,30 @@
+-----BEGIN CERTIFICATE-----
+MIIFMDCCAxigAwIBAgIBUjANBgkqhkiG9w0BAQsFADAxMQ8wDQYDVQQDEwZSb290
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+-----END CERTIFICATE-----

tests/data_files/dir4/cert83.crt

@@ -0,0 +1,30 @@
+-----BEGIN CERTIFICATE-----
+MIIFKTCCAxGgAwIBAgIBUzANBgkqhkiG9w0BAQsFADAxMQ8wDQYDVQQDEwZJbnQg
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+M8Wytap8IX3+XKdVxO8VxwLFt5gxIO6HWkNDRS4=
+-----END CERTIFICATE-----

tests/suites/test_suite_x509parse.data

@@ -390,6 +390,10 @@ 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)
+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 Parse Selftest
 depends_on:POLARSSL_MD5_C:POLARSSL_PEM_C:POLARSSL_SELF_TEST
 x509_selftest: