ZTS: Fix zpool_expand_001_pos

zpool_expand_001_pos was often failing due to not seeing autoexpand
commands in the zpool history. During testing, I found this to be
unreliable (sometimes the "online" wouldn't appear in zpool history)
and unnecessary, as we could simply check that the pool increased in
size.

This commit revamps the test to check for the expanded pool size
and corresponding new free space.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #13743