ZTS: improve output clarity of check_prop_source

Instead of just failing, indicate the expected and actual value and
source as a NOTE. Tests using this failed in an earlier version of
the changeset and this information helped find the cause.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Will Andrews <will@firepipe.net>
Closes #11517