Make LSUB test use the correct class

The class we use for the LSUB reply used to be derived from the class
that handles the LIST reply. The test case for the LSUB was however
badly written and used the LIST reply. This worked since the LSUB class
inherited from the LIST reply class. Separating them caused the test to
fail. Fix it by properly renaming the type of the variable to the
appropriate class.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>