Unit testing for pcp module.

Added unit tests to test code in def/ module.
Currently, AverageNodePowerHistory(...) assumes that the DRAM power
readings are being monitored. In addition, it also assumes that the
host servers are dual socket machines. However, this assumption might
not always hold true. Thus, added a TODO to fix this.

pcp/utils.go

@@ -41,6 +41,7 @@ func AverageNodePowerHistory(history *ring.Ring) float64 {
 		return 0.0
 	}
 
+	// TODO (pkaushik) handle cases when DRAM power is not being monitored.
 	count /= 4 // Two PKGs, two DRAM for all nodes currently.
 
 	return (total / count)