relax allocated-memory tests for libcxx

Summary:
[ Extend D17527441 changes from F15MapTest into F14SetTest ]
The current version of libc++ deallocates the existing capacity when calling operator=(initializer_list), even if the map is empty. This diff relaxes the expectations of F14's allocated memory tests (which fall back to the underlying STL std::unordered_map on some platforms) to accept this implementation.

Reviewed By: yfeldblum

Differential Revision: D17726849

fbshipit-source-id: 4757792a5aefe6d085b5d164ffe7b663f6078d9a

folly/container/test/F14SetTest.cpp

@@ -104,8 +104,10 @@ void runAllocatedMemorySizeTest() {
     s.reserve(5);
     EXPECT_GT(testAllocatedMemorySize(), 0);
     s = {};
+    if (preciseAllocInfo) {
       EXPECT_GT(testAllocatedMemorySize(), 0);
     }
+  }
   EXPECT_EQ(testAllocatedMemorySize(), 0);
   EXPECT_EQ(testAllocatedBlockCount(), 0);
 }