Play Framework -Dconfig.resource Not Working

I am using Play 2.3.9 for Scala. I ran my test using an alternative configuration i.e.

sbt -Dconfig.resource=test-application.conf test

Play cannot find this configuration even it was located at /conf. I tried without the .conf extension and placed it in different directories. In the end I used the known to be working application.conf and failed. Nothing seems to satisfy it. However, Play managed to pick the configuration file when I used -Dconfig.file=/path/to/test-application.conf'. What I am missing here with-Dconfig.resource? Does-Dconfig.resource` even work? Thanks.