add more test cases to improve the test code coverage #4

In this patch, the following module test coverage improved to 100%

ceilometer/alarm/evaluator/__init__

Since the module defines an abstract class, it is important to
change the abstract method body from using "pass" to comments so
that the test coverage can possibly to be ushed to 100%. If the
abstract method body is the "pass" statement, then the test code
coverage can not be possibly 100% since abstract class can not
be instantiated, thus can not be tested.

Change-Id: Icaac1e45a695544d9c22fd5d20b79616d2e4d504
Merge "add more test cases to improve the test code coverage #4"