Seek at the end of AOF after truncate call.

It is not clear if files open in append only mode will automatically fix
their offset after a truncate(2) operation. This commit makes sure that
we reposition the AOF file descriptor offset at the end of the file
after a truncated AOF is loaded and trimmed to the last valid command.
Better truncated AOF loading tests.

Now there are tests to write more data after loading a truncated AOF,
testing that the loaded data is correct, appending more, and testing
again.