On svn-auth-x509 branch, add unit tests for X.509 parser.

* subversion/tests/libsvn_subr/x509-test.c: New file
* build.conf: Add new C test to the build.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1606199 13f79535-47bb-0310-9956-ffa450edef68