folly: fix constexpr methods in RandomTest on macos

Summary:
the RNG max and min methods must be marked constexpr in order to
compile.  The macos compiler doesn't know to propagate the constexpr-ness from
numeric_limits::max on its own, so we have to be explicit (this also matches
the annotations in ThreadLocalPRNG in Random.h)

Reviewed By: yfeldblum

Differential Revision: D3576189

fbshipit-source-id: e4eeb3361d1c48f582dad5a52e35cae133f007a1

folly/test/RandomTest.cpp

@@ -55,10 +55,10 @@ TEST(Random, FixedSeed) {
       return 4; // chosen by fair dice roll.
                 // guaranteed to be random.
     }
-    static result_type min() {
+    static constexpr result_type min() {
       return std::numeric_limits<result_type>::min();
     }
-    static result_type max() {
+    static constexpr result_type max() {
       return std::numeric_limits<result_type>::max();
     }
   };