Change locale in XCTestCase

LocaleExtension.swift

private extension MyTests {
    private func setLocaleAsJP() {
        let original = class_getClassMethod(NSLocale.self, #selector(getter: NSLocale.current))!
        let swizzled = class_getClassMethod(NSLocale.self, #selector(NSLocale.myCurrentLocale))!
        method_exchangeImplementations(original, swizzled)
    }
}

// https://stackoverflow.com/questions/31065859/how-can-i-change-the-locale-on-the-xcode-playground
fileprivate extension NSLocale {
    @objc class func myCurrentLocale() -> NSLocale {
        return NSLocale(localeIdentifier: "ja")
    }
}