Exercise new taskq interface in splat-taskq tests

The splat-taskq test functions were slightly modified to exercise
the new taskq interface in addition to the old interface.  If the
old interface passes each of its tests, the new interface is
exercised.  Both sub tests (old interface and new interface) must
pass for each test as a whole to pass.

Signed-off-by: Prakash Surya <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #65