Further limit the sparse file testing on OS X

At 4GiB the CI test system still spends a considerable
amount of time testing this. Tune the size down to
16 MiB.

Change-Id: I417aa6829fcc734e5de4d7d34e503190f6b291e5
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

tests/auto/corelib/io/largefile/tst_largefile.cpp

@@ -75,7 +75,7 @@ public:
     #elif defined(Q_OS_MAC)
         // HFS+ does not support sparse files, so we limit file size for the test
         // on Mac OS.
-        maxSizeBits = 32; // 4 GiB
+        maxSizeBits = 24; // 16 MiB
     #else
         maxSizeBits = 24; // 16 MiB
     #endif