Create standard, fast tests for substraction, fixed order and powheg events

[Bijan Chokoufe Nejad]

For this we need a dummy_virtual that gives an array of matrix elements = 1. Results don't have to be physical but reproducable. For the events we can use the trimmed down version of the debug output. For powheg, we need one test that samples a bit of the grid and one test that uses an existing grid and generates an event.

[Juergen Reuter]

As of now, tests are done for subtraction terms and fixed order events, but not yet for POWHEG events. There are two more tests intended to come.

[Bijan Chokoufe Nejad]

Added in r7224. It turned out that the dummy virtuals were fast enough to make the generation of a POWHEG event one test (still under 20 sec). Added a test nlo_3 to also test the generation of an NLO event in debug format.