ZTS: Wait for free space between write_dirs tests

Cleanup for write_dirs involves destroying a dataset filling a pool
and then recreating the dataset for the next test. Due to the
asynchronous nature of free space accounting, recreating the dataset
can fail for lack of space, causing problems for the next test.

Add wait_freeing $TESTPOOL to wait for the space to be freed and then
sync_pool $TESTPOOL to update the space accounting before attempting
to recreate the test filesystem.

Only use a single disk to create the pool. Make it a small file so it
does not take too long to fill.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10112