tests: lib: notify: Fix uninitialised variable warning

This commit sets an initial value of 0 for the `res` variable, whose
pointer is passed to other functions and is not directly assigned
within the calling function.

Note that, when the test completes successfully, the value of the `res`
variable should be set to `423` (the value of `set_res`).

This fixes the "‘res’ may be used uninitialized" warning generated by
the GCC 12.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
(cherry picked from commit bde270b1)