hdf5_msb

Make tests pass on both of my branches, like they ought to: in the tests that
call H5F__vfd_swmr_writer_md_test(), actually sleep for more than `max_lag`
ticks so that the deferred-frees queue is actually flushed before we force new
items onto the queue and count them.

In H5F__vfd_swmr_writer_md_test(), check that the number of deferred
shadow-space frees is *precisely* the number `nshadow_defrees`, since that
seems to be what the tests that call this routine intend.