Custom window merging

[Etienne Chauchot]

Hi all,

There is now 2 new ValidatesRunner tests: 
WindowTest.testMergingCustomWindows and 
WindowTest.testMergingCustomWindowsKeyedCollection. The aim of these 
tests is to verify that the runners can handle custom windowFn 
(extensions of windowFn that, for example, could rely on elements in 
addition to timestamps).

As new runners are coming, I wanted to let you know that there is also a 
new category tag UsesCustomWindowMerging that you can use to skip these 
tests while running ValidatesRunner tests on runners that do not support 
custom window merging yet.

Besides, there is also an ongoing related PR 
(https://github.com/apache/beam/pull/3592) to enhance the test utils 
methods of WindowFnTestUtils.

Etienne