Temporarily disable the reguid test.

Currently, ztest fails with the following error:

error: Pool 'ztest' has encountered an uncorrectable I/O failure
and the failure mode property for this pool is set to panic.

We know how to fix it (see issue #939), but it may take some time
before we get around to merging the fix, which has some heavy
dependencies.

In the mean time, it is not ideal to be unable to use ztest just
because of a small isolated issue, so this patch works around the
problem by disabling the reguid test. This is just a temporary hack to
keep ztest usable.

The reguid test will be enabled again when the proper fix is merged.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #997