enable logging to stderr by default in test binaries

Summary:
Update folly's TestMain program to enable glog logging to stderr by default.

Some of the tests in folly/experimental/test/TestUtilTest.cpp require that this
is enabled, and fail if log messages are not logged to stderr.

Reviewed By: yfeldblum

Differential Revision: D6817543

fbshipit-source-id: 87724c52124e72ad19f65524bcbd6536680f02ec

folly/test/common/TestMain.cpp

@@ -16,6 +16,8 @@
 
 #include <folly/init/Init.h>
 
+#include <folly/Portability.h>
+#include <folly/portability/GFlags.h>
 #include <folly/portability/GTest.h>
 
 /*
@@ -26,7 +28,14 @@
 int main(int argc, char** argv) __attribute__((__weak__));
 
 int main(int argc, char** argv) {
+#if FOLLY_HAVE_LIBGFLAGS
+  // Enable glog logging to stderr by default.
+  gflags::SetCommandLineOptionWithMode(
+      "logtostderr", "1", gflags::SET_FLAGS_DEFAULT);
+#endif
+
   ::testing::InitGoogleTest(&argc, argv);
   folly::Init init(&argc, &argv);
+
   return RUN_ALL_TESTS();
 }