Add certificate files for NSS AIA fetching unittest

net/data/ssl/certificates/README

 This directory contains various certificates for use with SSL-related
 unit tests.
 
 - google.binary.p7b
 - google.chain.pem
 - google.pem_cert.p7b
 - google.pem_pkcs7.p7b
 - google.pkcs7.p7b
 - google.single.der
 - google.single.pem
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 - duplicate_cn_2.p12
 - duplicate_cn_2.pem
      Two certificates from the same issuer that share the same common name,
      but have distinct subject names (namely, their O fields differ). NSS
      requires that certificates have unique nicknames if they do not share the
      same subject, and these certificates are used to test that the nickname
      generation algorithm generates unique nicknames.
      The .pem versions contain just the certs, while the .p12 versions contain
      both the cert and a private key, since there are multiple ways to import
      certificates into NSS.
+
+- aia-cert.pem
+- aia-intermediate.der
+- aia-root.pem
+     A certificate chain which we use to ensure AIA fetching works correctly
+     when using NSS to verify certificates (which uses our HTTP stack).
+     aia-cert.pem has a caIssuers that points to "aia-test.invalid" as the URL
+     containing the intermediate, which can be served via a URLRequestFilter.
+     aia-intermediate.der is stored in DER form for convenience, since that is
+     the form expected of certificates discovered via AIA.