Mocking a static class method in a swift unit test

I'm a seasoned Objective-c programmer but I can't say the same for Swift, I'm having a hard time unit testing a class in swift without using frameworks like OCMock.

The Problem: I'm integrating Firebase into a mixed Objective-C/Swift project, and I need to configure it based on the build configuration of the app.

I've written a Swift class for that (that will be used by the obj-c app delegate), however since the firebase framework is configured trough a static class method, precisely FIRApp.configure(with: FIROptions), I need to mock this method somehow in order to unit test it.

My code, without any handle for Dependency Injection, looks like that:

@objc class FirebaseConfigurator: NSObject{

    func configureFirebase(){

        let config = configManager.buildConfiguration

        var optionsPlistBaseName = getPlistName()

        let optionsFile = Bundle.main.path(forResource: optionsPlistBaseName, ofType: "plist")

        guard let opts = FIROptions(contentsOfFile: optionsFile) else{
            assert(false, "fatal: unable to load \(optionsFile)")
            return
        }

        FIRApp.configure(with: opts)

    }

    func getPlistName() -> String{
        // retrieves correct plist name and returns it
    }

}

I've done some research but so far I didn't find nothing that fits my solution, however I was thinking of one of the following:

I could pass a function that defaults to FIRApp.configure(with:) however I should do this from objective-c and the function also accepts a parameter, I was struggling with the syntax
I could use a wrapper around FIRApp, but I wanted to avoid it unless the only viable clean solution.
I could keep on playing with protocols and do dependency inversion, however being the method static I was struggling with the syntax again, I can't find an easy way to do DI with a mock class with a static method.

As a reference (both personal and for who might need it) these are some of the resources I found useful and upon which I will keep on digging:

Dealing with static cling in Swift
This Question
This article about generic unit testing

In the meanwhile, every help would be really appreciated.

As a sidenote, there are many ways I can solve this problem without struggling with mocking a static class method, but my aim here is to find out a way of mocking it in order to have a better understanding of the best practices when testing more complex situations.

You can indeed do any of those.

Closure Argument

You can have your configureFirebase function take an "applier" closure that defaults to what you originally used:

func configureFirebase(
    using apply: (_ options: FIROptions) -> Void
        = { opts in FIRApp.configure(opts) }
) {
  // building |opts| as before
  // Now replace this: FIRApp.configure(with: opts)
  apply(opts)
}

Protocols

You need a Configurable protocol, and then to conform FIRApp to it for the default case:

protocol Configurable {
  static func configure(with options: FIROptions)
}

extension FIRApp: Configurable {}

class FirebaseConfigurator {
  var configurable: Configurable
  init(configurable: Configurable = FIRApp) {
    self.configurable = configurable
  }

  func configureFirebase() {
    //load |opts|…
    configurable.configure(with: opts)
  }
}

If you're just going to use this in one method, though, it's merely transient state, and it should probably be a function argument rather than stored property.

(If it's unclear whether it's persistent or transient state because the whole point of the class is to call a single function, perhaps you don't even need a class, just a function.)