Fix test_set_zero_periodic unit test failure

Removed an "EXPECT_EQ(cb_counter, 0)" statement
right after a timer of value 0 is scheduled.
The EXPECT_EQ statement shouldn't be there,
because by the time it is reached, the callback might
have been called already.

Change-Id: I61fbec96c436787321c6b36006a2bc1c0e627667