Bug fix for superslab meta-data (#302)

* Replace time measuring macro

The DO_TIME macro was used originally to get performance numbers. The
macro makes tests hard to debug. This commit replaces it with a proper
C++ class with destructor.

* Bug fix

If the superslab meta data is large, then the calculation for the
sizeclasses that could use the short slab was incorrect.  This fixes
that calculation.

Co-authored-by: Nathaniel Wesley Filardo <nfilardo@microsoft.com>

src/test/perf/external_pointer/externalpointer.cc

@@ -64,7 +64,9 @@ namespace test
 #endif
     setup(r, alloc);
 
-    DO_TIME("External pointer queries ", {
+    {
+      MeasureTime m;
+      m << "External pointer queries ";
       for (size_t i = 0; i < iterations; i++)
       {
         size_t rand = (size_t)r.next();
@@ -77,7 +79,7 @@ namespace test
         if (calced_external != external_ptr)
           abort();
       }
-    });
+    }
 
     teardown(alloc);
   }