Test by using protocols

LoadContentVC3Test.swift

import XCTest
import Foundation

/// File manager stub to override the methods we need
class FileManagerStub: AppFileManager {
  func getContent(atPath: Stirng) -> Data? {
    return Data()
  }
}

/// Class under test
class MyViewControllerTest: XCTestCase {
  
  func testLoadContent_withEmptyFile() {
    let vc = MyViewController()
    let fileManagerStub = FileManagerStub()
    let decoder: JSONDecoder = JSONDecoder()
    vc.loadContent(fileManager: fileManagerStub, path: "emptyFile", jsonDecoder: decoder)
    XCTAssertTrue(vc.content.isEmpty)    
  }
}