Fix tests: NULL pointer dereference in ust channel unit tests

The test_create_ust_channel() test case erroneously checks for
a NULL session instead of a channel. This can result in a
NULL pointer dereference on failure to create a ust channel.

The scope of usess is reduced to prevent similar mistakes in the
future. Moving 'dom' has made it obvious that this variable is
unused. Hence, it is removed.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>