Add gtest, as separate set of tests (that just follow the ad-hoc tests that already exist)

test/main.cpp

@@ -1,6 +1,9 @@
 #include "tests.h"
 
-int main() {
+#include "gtest/gtest.h"
+
+int main(int argc, char** argv) {
+  ::testing::InitGoogleTest(&argc, argv);
   Test::RunAll();
-  return 0;
+  return RUN_ALL_TESTS();
 }