tests/block_cloning: try harder to stay on same txg in fallback test

We've observed this test failing intermittently. When it does, the
"same block" check shows that both files have the same content, that is,
the file was cloned.

The only way this could have happened is if the open txg moved between
the dd and clonefile calls. That's possible because although we set
zfs_txg_timeout to be large, that only affects the wait time in the sync
thread at the start of a new txg; it doesn't change anything if its
currently waiting or working.

So here we just force the txgs to move immediately before, which should
get both operations onto the same txg as intented.

Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris Rob Norris <rob.norris@klarasystems.com>
Closes #15303