Duplicate tests in test_sort::test_buffer_sizes

[Nahor]

The test for "1000G" and "10T" are repeated for each of the smaller buffer sizes because of an incorrectly placed curly brace:
(error introduced by commit 6b8de1d)

for buffer_size in <small buffers> {
   <test small buffer>
   
   {
     <test big buffers>
   }
}

[cakebaker]

Good catch, thanks!