tdb/test: fix up tests for use in SAMBA tdb code.

tdb/test: fix up tests for use in SAMBA tdb code.

1) Make sure we include "tdb_private.h" first, to get the right headers
   (esp. the correct setting of _FILE_OFFSET_BITS before unistd.h).
2) Fix 3G file test since expand logic has changed.
3) Fix nested transaction test, since default is to allow nesting.
4) Capture fdatasync, which was slowing down transaction expand.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>