How to subclass NSLocale for tests

I want to unit test some Swift classes which would take Locale as a parameter and then behave in locale-specific way.

It looks like there is no easy way to provide my own locale implementation though. I'd like to avoid swizzling, so I tried to subclass NSLocale, i.e.:

class MyLocale: NSLocale {
   init(myargs: whatever) {
      // initialize here
      super.init(localeIdentifier: "C")
   }
}

but this crashes in runtime:

 -[NSLocale initWithLocaleIdentifier:]: method only defined for abstract class.

So I re-define that initializer:

class MyLocale: NSLocale {
   override init(localeIdentifier string: String) {
      // initialize here
      super.init(localeIdentifier: string) // putting super.init() here doesn't help either
   }
}

but the result is the same: but this crashes in runtime:

 -[NSLocale initWithLocaleIdentifier:]: method only defined for abstract class.

is there any way to properly do what I want without resorting to Objective-C runtime and method swizzling?