validation test papi_l2_dcw passes even when no cache detected

[Al Grant]

If cache discovery fails (e.g. due to #128), validation_tests/papi_l2_dcw carries on anyway, creating a 0-sized array of doubles, and reporting PASSED.

Actual failure of cache discovery (e.g by some error condition within cache discovery) should be caught by the test and report FAILED. The combination of apparently successfully detecting a system without L2 cache, but having an L2 preset event, should probably report SKIPPED at best. Reporting PASSED on the basis of not actually running the test stimulus at all, is just wrong.