In Swift, you can call any instance method as a static function and it will return a closure representing that method. This is how running tests using SPM on Linux works.

// This produces a '() -> Void' closure which is a reference to the
// given view's 'removeFromSuperview' method.
let closure = UIView.removeFromSuperview(view)

// We can now call it just like we would any other closure, and it
// will run 'view.removeFromSuperview()'
closure()

// This is how running tests using the Swift Package Manager on Linux
// works, you return your test functions as closures:
extension UserManagerTests {
    static var allTests = [
        ("testLoggingIn", testLoggingIn),
        ("testLoggingOut", testLoggingOut),
        ("testUserPermissions", testUserPermissions)
    ]
}