[ios] Add test for code snippet added to MGLPolyline doc change

platform/darwin/src/MGLPolyline.h

@@ -43,9 +43,8 @@ NS_ASSUME_NONNULL_BEGIN
      CLLocationCoordinate2D(latitude: 35.68476, longitude: -220.24257),
      CLLocationCoordinate2D(latitude: 37.78428, longitude: -122.41310)
  ]
- 
- let polyline = MGLPolyline(coordinates: &coordinates, count: UInt(coordinates.count))
-```
+ let polyline = MGLPolyline(coordinates: coordinates, count: UInt(coordinates.count))
+ ```
 
  A polyline is known as a
  <a href="https://tools.ietf.org/html/rfc7946#section-3.1.4">LineString</a>

platform/darwin/test/MGLDocumentationExampleTests.swift

@@ -103,6 +103,18 @@ class MGLDocumentationExampleTests: XCTestCase, MGLMapViewDelegate {
 
         XCTAssertNotNil(mapView.style?.source(withIdentifier: "pois"))
     }
+    
+    func testMGLPolyline() {
+        //#-example-code
+        let coordinates = [
+            CLLocationCoordinate2D(latitude: 35.68476, longitude: -220.24257),
+            CLLocationCoordinate2D(latitude: 37.78428, longitude: -122.41310)
+        ]
+        let polyline = MGLPolyline(coordinates: coordinates, count: UInt(coordinates.count))
+        //#-end-example-code
+        
+        XCTAssertNotNil(polyline)
+    }
 
     func testMGLCircleStyleLayer() {
         let population = MGLVectorSource(identifier: "population", configurationURL: URL(string: "https://example.com/style.json")!)