CVE-2017-9788: add unit tests for get_digest_rec()

Including the module source is a dirty hack, but maybe the direct way is
best for now. More functional tests are still TODO.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpdunit@1801996 13f79535-47bb-0310-9956-ffa450edef68