Add tests for LazyKSubsets and LazyPowerset

Add tests for LazyKSubsets and LazyPowerset

The LazyKSubsets and LazyPowerset are simply
wrappers around an iterator type which can
enumerate all subsets of a given iteration.

While this won't be used anymore to enumerate
all possible sets of events of the configuration
in the compatibility graph, which will soon be
removed, it will still be used (or a variant
thereof) when computing K-partial alternatives
later on