test_server_split_with_indexes: close packwriter

test_server_split_with_indexes: close packwriter

Otherwise there are random test failures when the tmpdir is removed
before the PackWriter's destructor calls close().  This doesn't cause an
immediate problem, but it adds an unexpected entry to saved_errors that
causes later tests to fail.

The related saved_errors entries looked like this:

  ['../../../bup', 'midx', '--auto', '--dir', ...]: returned 15

and the underlying (hidden) midx stderr output in that case looked like
this:

  error: '...' is not a bup repository; run "bup init"

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>