DSC - Acceptance and Integration Tests Run Slow

The initial fix for MODULES-2977 introduced a significant performance penalty which can be avoided. It was decided to defer the performance improvement until later so that the module can be released.

The issue is that the "get_dsc_vendor_resource_abs_path" method in the "dsc_utlis.rb" library always looks up the Puppet "libdir" path for every invocation. This can be avoided by caching the libdir for known clients for the duration of the test run.