Add test cases for `Concat` and `Merge` flattening strategies.
Add test cases for `Concat` and `Merge` flattening strategies.

ReactiveCocoa/ReactiveSwift,walkingsmarts/ReactiveCocoa,andersio/ReactiveCocoa,sandyway/ReactiveCocoa,ikesyo/ReactiveCocoa,calebd/ReactiveCocoa,walkingsmarts/ReactiveCocoa,victorlin/ReactiveCocoa,ohwutup/ReactiveCocoa,walkingsmarts/ReactiveCocoa,victorlin/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ohwutup/ReactiveCocoa,calebd/ReactiveCocoa,victorlin/ReactiveCocoa,ikesyo/ReactiveCocoa,sandyway/ReactiveCocoa,andersio/ReactiveCocoa,andersio/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,sandyway/ReactiveCocoa