Don't use private googletest API in TestUtils.h

Summary:
Until [very recently](google/googletest#2517), `googletest` did not print the skip message in any shape or form, so nothing can possibly depend on us customizing the skip message here.

The default is perfectly fine, let's keep it simple.

Reviewed By: yfeldblum

Differential Revision: D18051635

fbshipit-source-id: f7b889b0adf5e34a70c2579a34748bb159809b3c

folly/test/TestUtils.h

@@ -76,7 +76,7 @@
 //     that can identify the "Test skipped by client" in the failure message
 //     and convert this into a skipped test result.
 #ifdef GTEST_SKIP_
-#define SKIP(msg) GTEST_SKIP_("Test skipped by client")
+#define SKIP(msg) GTEST_SKIP()
 #else
 #define SKIP()                                       \
   GTEST_AMBIGUOUS_ELSE_BLOCKER_                      \