Handle tmp dir on Android

Summary:
There is no universally accessible tmpdir on Android which
means mkstemp fails. This diff calls a function that the test runner
should provide to fill a valid temporary directory.

Reviewed By: yangchi

Differential Revision: D2842034

fb-gh-sync-id: 9b826757bd750af016a18adccd5a21174be644d6

folly/test/FBStringTest.cpp

@@ -33,6 +33,7 @@
 #include <folly/Portability.h>
 #include <folly/Random.h>
 #include <folly/Conv.h>
+#include <folly/experimental/TestUtil.h>
 
 using namespace std;
 using namespace folly;