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.
Had to replace "\\" with "\\\\" to retain the backslash(\).
The TargetRegexBuilder when building the patterns from the values of workspace and feed resource attributes, it was removing the backslash( for example, the backslash in the regex "usagetest\d{1,2}" ). To avoid this and to have a backslash literally in the regex, I had to replace "\\" with "\\\\".