Clean-up: tests: notification-client: unused assignment

1452629 Unused value

An assigned value that is never used may represent unnecessary
computation, an incorrect algorithm, or possibly the need for cleanup or
refactoring.

In main: A value assigned to a variable is never used. (CWE-563)

CID 1452629 (#1 of 1): Unused value (UNUSED_VALUE)assigned_value:
Assigning value -1 to ret here, but that stored value is overwritten
before it can be used

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I0d80a244a7b2b3d6aed57d58ca5435230d664f99