ZTS: Waiting for zvols to be available

ZTS: Waiting for zvols to be available

The ZTS block_device_wait helper function should use -e when waiting
for a file to appear since it will be either a block special device
or a symlink.  This didn't cause any failures but when a device path
was specified the function would wait longer than needed.

Additionally update the most flakey test cases to pass the file path
to block_device_wait to try and improve the test reliability.  The
udev behavior on Fedora in particular can result in frequent false
positives.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12515