B-78838: enable regex to support usagetest feeds

-  modified the TargetRegexBuilder custom code to retain the backslashes in the workspace resource regular expression and feed regular expression after reading them from the configuration file atom-server.cfg.xml
 - added a testcase.
Sorry, I updated my comment.  Github ate one of my slashes in the second double-slash.

What I mean, is that rather than hardcoding double-slash to quadruple slash, have the replace be slash to double-slash.