Signal lifetime: added test to verify that observe handlers are deallocated.
Signal lifetime: added test to verify that observe handlers are deallocated.

Brought up by @CosynPa, I realized that it's a pretty important test to have.
Fixes #2698.

calebd/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,andersio/ReactiveCocoa,calebd/ReactiveCocoa,sandyway/ReactiveCocoa,kevin-zqw/ReactiveCocoa,victorlin/ReactiveCocoa,kevin-zqw/ReactiveCocoa,walkingsmarts/ReactiveCocoa,smilypeda/ReactiveCocoa,tzongw/ReactiveCocoa,andersio/ReactiveCocoa,Juraldinio/ReactiveCocoa,ohwutup/ReactiveCocoa,andersio/ReactiveCocoa,sandyway/ReactiveCocoa,smilypeda/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,Juraldinio/ReactiveCocoa,rpowelll/ReactiveCocoa,kevin-zqw/ReactiveCocoa,victorlin/ReactiveCocoa,ohwutup/ReactiveCocoa,ikesyo/ReactiveCocoa,sandyway/ReactiveCocoa,smilypeda/ReactiveCocoa,walkingsmarts/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,victorlin/ReactiveCocoa,tzongw/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,rpowelll/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ikesyo/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,rpowelll/ReactiveCocoa,tzongw/ReactiveCocoa,Juraldinio/ReactiveCocoa