add snapshot test

[GeekTree0101]

Before run unit test, you must set recordMode as true.

class KarrotFlexSnapshotTestCase: FBSnapshotTestCase {

  var testView: KarrotFlexTestView!

  override func setUp() {
    super.setUp()
    self.testView = KarrotFlexTestView()
    self.testView.backgroundColor = .green
    self.recordMode = false // <- HERE
  }

  func verify() {
    FBSnapshotVerifyView(self.testView)
  }

  func testDefine(parentSize: CGSize, _ closure: @escaping (Flex) -> Void) {
    self.testView.flex.define(closure)
    self.testView.bounds = CGRect(origin: .zero, size: parentSize)
    self.testView.flex.layout()
    self.testView.setNeedsLayout()
  }
}

[GeekTree0101]

After merging it first, I will take it in the direction of supplementing the snapshot test.