util: SumTree test build fix

This fixes a "funny" conflict between the SumTreeTest and
weather-monitor's experiment.hpp.

Note to self: Preprocessor macros are evil.

Change-Id: I3f8c95fe086357db77110c0c53d3120ca839f30a

src/core/util/testing/SumTreeTest.cc

@@ -1,10 +1,12 @@
 #include "util/SumTree.hpp"
+#include "util/Logger.hpp"
 
 #include <iostream>
-#define LOG std::cerr
 
 using std::endl;
 
+fail::Logger LOG("SumTreeTest");
+
 struct Pilot {
 	uint32_t id;
 	uint32_t instr2;