further relax F14 allocated memory size test for libc++

Summary:
F14MapTest already had properly relaxed the post-condition for
fallback maps after a move-clear and reserve; this diff applies the same
relaxation to F14SetTest.

Reviewed By: shixiao

Differential Revision: D9756311

fbshipit-source-id: 8285bb05023d923c4a7c4e014c3668bdaed591c4

folly/container/test/F14SetTest.cpp

@@ -97,9 +97,9 @@ void runAllocatedMemorySizeTest() {
     EXPECT_EQ(testAllocatedBlockCount, emptySetAllocatedBlockCount);
 
     s.reserve(5);
-    EXPECT_GT(testAllocatedMemorySize, emptySetAllocatedMemorySize);
+    EXPECT_GT(testAllocatedMemorySize, 0);
     s = {};
-    EXPECT_GT(testAllocatedMemorySize, emptySetAllocatedMemorySize);
+    EXPECT_GT(testAllocatedMemorySize, 0);
   }
   EXPECT_EQ(testAllocatedMemorySize, 0);
   EXPECT_EQ(testAllocatedBlockCount, 0);