Issue 2715 - Speed up notification target tests

Issue 2715 - Speed up notification target tests

These tests were pretty heavy. There's some merit in testing all possible
combinations, but the tests ran for 40 seconds on my machine.

I've made three changes that brought the runtime down to 3 seconds (on my
machine):

1. Move target combination tests into a separate test and reduce the number of
   targets. The target selection test already covers the edge cases of target
   selection - there is no real need to repeat this for all target combinations.
   One matching and one non-matching value for each property should suffice.
2. Don't generate pairs of the same target - doesn't really make sense and saves
   a bit of runtime.
3. Don't test for information vs critical for all target combinations. That
   critical notifications get precedence is covered by other tests, and not
   testing it for all target combinations cuts the test runtime in half.

[Wladimir Palant, 2015-11-02 20:17:53]

LGTM