ZTS: Fix zpool_reguid Makefile.am and tests

The zpool_reguid tests were not being included the dist tarball
resulting in them not running. This is reported as a "failed
verification" warning by the CI. Add the tests to the correct
Makefile.am.

Additionally, remove the usage of 'bc -e <expr>' from the tests.
This option is only supported by the FreeBSD version of bc.

Update the test case to reflect the 0 is not a valid GUID.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16559