ZTS: Fix redacted_send failures on FreeBSD

We're seeing failures for redacted_deleted and redacted_mount
on FreeBSD 13-15:

    09:58:34.74 diff: /dev/fd/3: No such file or directory
    09:58:34.74 ERROR: diff /dev/fd/3 /dev/fd/4 exited 2

The test was trying to diff the file listings between two directories to
see if they are the same. The workaround is to do a string comparison
of the directory listings instead of using diff.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #16224