fixed 3 tests not imported needing types

[IgorRatynski]

Steps for error reproduction: clone the repo, open Package.swift file, launch tests (by command+u) and after you'll have an error - "Cannot find type 'CGSize' in scope" in "SignalProducer+MoyaSpec", "Single+MoyaSpec" and "Observable+MoyaSpec" files

[antranapp]

Have a similar MR to fix the same things

@AndrewSB Any chance this fix can be merged?

[thejohnlima]

@IgorRatynski as I commented here, all tests are working fine in the latest version of Xcode. So, in my opinion this PR is not required.

[AndrewSB]

hey sorry, just seeing this now. mention me again if this is needed for tests to compile