Always run-test-conf, but skip if not built.
The test-conf test requires libjson-c to be available in order to be built. However, there has been no user indication that additional tests could be built if the json-c development files were available.
Continue to not build test-conf if json-c is not available, but do run the test harness. The test harness is updated to SKIP the test if the test-conf binary is unavailable.