XCTest extension from another file

I am new to Xcode XCTest and i want to test my Extension from another file

StringExtensions.swift

import Foundation
extension String : IntegerLiteralConvertible{
    public init(integerLiteral value: Int) {
       self = "\(value)"
    }
}

StringExtensionsTest.swift

import XCTest
class StringExtensionsTest: XCTestCase {

override func setUp() {
    super.setUp()
}

override func tearDown() {
    super.tearDown()
}

func testIntegerInitializer() {
    let string:String = 12345
    assert(string == "12345")
}

}

And i get "Cannot convert value of type 'Int' to specified type 'String'" but when i put this extension into Test file it works like a charm.

How can I test extensions from another file?