lightbox test [nfc]: Tighten test cases using a checkPositionsRelative

This makes some of these test cases quite a bit shorter, without
losing any information that's relevant to what the test is about.

Partly that comes by removing some tokens (like `kTestVideoDuration`)
that were repetitive and not the interesting aspect of the test.

Partly it comes by then as a result being able to collapse three lines
into one line at each of these steps.  That brings the different parts
of the test closer together and makes them easier to see all at once.

Author: gnprice

test/widgets/lightbox_test.dart

@@ -240,6 +240,16 @@ void main() {
       check(platform.position).equals(video);
     }
 
+    /// Like [checkPositions], but expressed in units of [kTestVideoDuration].
+    void checkPositionsRelative(WidgetTester tester, {
+      required double slider,
+      required double video,
+    }) {
+      checkPositions(tester,
+        slider: kTestVideoDuration * slider,
+        video: kTestVideoDuration * video);
+    }
+
     (Offset, Offset) calculateSliderDimensions(WidgetTester tester) {
       const padding = 24.0;
       final rect = tester.getRect(find.byType(Slider));
@@ -306,9 +316,7 @@ void main() {
 
       await tester.pump(kTestVideoDuration * 0.5);
       platform.pumpEvents();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.5,
-        video: kTestVideoDuration * 0.5);
+      checkPositionsRelative(tester, slider: 0.5, video: 0.5);
       check(platform.isCompleted).isFalse();
       check(platform.isPlaying).isTrue();
 
@@ -324,18 +332,14 @@ void main() {
       // At exactly the end of the video.
       await tester.pump(const Duration(milliseconds: 500));
       platform.pumpEvents();
-      checkPositions(tester,
-        slider: kTestVideoDuration,
-        video: kTestVideoDuration);
+      checkPositionsRelative(tester, slider: 1.0, video: 1.0);
       check(platform.isCompleted).isTrue(); // completed
       check(platform.isPlaying).isFalse(); // stopped playing
 
       // After the video ended.
       await tester.pump(const Duration(milliseconds: 500));
       platform.pumpEvents();
-      checkPositions(tester,
-        slider: kTestVideoDuration,
-        video: kTestVideoDuration);
+      checkPositionsRelative(tester, slider: 1.0, video: 1.0);
       check(platform.isCompleted).isTrue();
       check(platform.isPlaying).isFalse();
     });
@@ -350,33 +354,23 @@ void main() {
       // gesture increments.
       final gesture = await tester.startGesture(trackStartPos);
       await tester.pump();
-      checkPositions(tester,
-        slider: Duration.zero,
-        video: Duration.zero);
+      checkPositionsRelative(tester, slider: 0.0, video: 0.0);
 
       await gesture.moveBy(trackLength * 0.2);
       await tester.pump();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.2,
-        video: Duration.zero);
+      checkPositionsRelative(tester, slider: 0.2, video: 0.0);
 
       await gesture.moveBy(trackLength * 0.4);
       await tester.pump();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.6,
-        video: Duration.zero);
+      checkPositionsRelative(tester, slider: 0.6, video: 0.0);
 
-      await gesture.moveBy(-trackLength * 0.2);
+      await gesture.moveBy(trackLength * -0.2);
       await tester.pump();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.4,
-        video: Duration.zero);
+      checkPositionsRelative(tester, slider: 0.4, video: 0.0);
 
       await gesture.up();
       await tester.pump();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.4,
-        video: kTestVideoDuration * 0.4);
+      checkPositionsRelative(tester, slider: 0.4, video: 0.4);
 
       // Verify seekTo is called only once.
       check(platform.callLog.where((v) => v == 'seekTo').length).equals(1);
@@ -390,21 +384,15 @@ void main() {
 
       final gesture = await tester.startGesture(trackStartPos);
       await tester.pump();
-      checkPositions(tester,
-        slider: Duration.zero,
-        video: Duration.zero);
+      checkPositionsRelative(tester, slider: 0.0, video: 0.0);
 
       await gesture.moveBy(trackLength * 0.5);
       await tester.pump();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.5,
-        video: Duration.zero);
+      checkPositionsRelative(tester, slider: 0.5, video: 0.0);
 
       await gesture.up();
       await tester.pump();
-      checkPositions(tester,
-        slider: kTestVideoDuration * 0.5,
-        video: kTestVideoDuration * 0.5);
+      checkPositionsRelative(tester, slider: 0.5, video: 0.5);
 
       final basePosition = kTestVideoDuration * 0.5;
       Duration actualElapsed = basePosition;
@@ -424,9 +412,7 @@ void main() {
         if (actualElapsed.inMilliseconds % 500 == 0) {
           lastPolled += const Duration(milliseconds: 500);
         }
-        checkPositions(tester,
-          slider: lastPolled,
-          video: actualElapsed);
+        checkPositions(tester, slider: lastPolled, video: actualElapsed);
       }
     });
   });