title: UserDefaults not working on Linux
body: <p><code>UserDefaults</code> does not appear to function in an XCTest when running on Linux. I'm running the below test but <code>userDefaults.string(forKey: "key")</code> returns <code>nil</code> on Linux when on macOS it returns the correct value and passes.</p>
  
  <p>I'm using Docker to execute the tests on Linux. My Dockerfile is as follows:</p>
  
  <pre><code>FROM swiftdocker/swift
  
  WORKDIR /package
  
  COPY . ./
  
  RUN swift package resolve
  RUN swift package clean
  RUN swift test --parallel
  </code></pre>
  
  <p>The test file:</p>
  
  <pre><code>import XCTest
  
  class UserDefaultsTests: XCTestCase {
  
      func testUserDefaults() {
  
          let userDefaults = UserDefaults.standard
  
          userDefaults.set("value", forKey: "key")
  
          _ = userDefaults.synchronize()
  
          XCTAssertEqual(userDefaults.string(forKey: "key"), "value") //nil is not equal to "value"
      }
  
      static var allTests = [
          ("testUserDefaults", testUserDefaults)
      ]
  }
  </code></pre>
  
  <p><code>UserDefaults</code> appears to be <a href="https://github.com/apple/swift-corelibs-foundation/blob/master/Foundation/UserDefaults.swift" rel="nofollow noreferrer">implemented in Swift Core Libs here</a>, which is available on Linux, so I'm not sure why it wouldn't work.</p>