@Override
    protected Fragment createFragment(long addr, int size) {
      return new TestableFragment(addr, size);
    }
Just like Fragment except that the first time allocate is called it returns false indicating
that the allocate failed. In a real system this would only happen if a concurrent allocate
happened. This allows better code coverage.
geode-core/src/test/java/org/apache/geode/internal/offheap/FreeListManagerTest.java