Answer
At the time this was investigated, Developer Studio's unit-testing tooling didn't actually support .pltst files for CUST-layer customizations — the confusing mixed signal (a "no unit tests" message alongside real, working pass/fail results) reflected that gap rather than a bug in your specific test or package. Check current Developer Studio release notes for whether CUST-layer unit-test support has since been added, since this was an acknowledged, tracked limitation rather than a permanent design decision.