Tag the video element with its track source for e2e specs

The SDK MediaView renders a plain video element, so LiveKit's
data-lk-source attribute is gone. Expose data-source from the SDK track
and point the screen-share spec at it.

playwright/widget/screen-share.test.ts

@@ -76,9 +76,9 @@ widgetTest("Sharing screen in group call", async ({ addUser, browserName }) => {
       timeout: 5000,
     });
 
-    await expect(
+    await expect(frame.locator('video[data-source="screenShare"]')).toHaveCount(
-      frame.locator('video[data-lk-source="screen_share"]'),
+      1,
-    ).toHaveCount(1);
+    );
   }
 
   // Alice should be in grid mode as she is local sharing
@@ -117,9 +117,9 @@ widgetTest("Sharing screen in group call", async ({ addUser, browserName }) => {
       timeout: 5000,
     });
 
-    await expect(
+    await expect(frame.locator('video[data-source="screenShare"]')).toHaveCount(
-      frame.locator('video[data-lk-source="screen_share"]'),
+      2,
-    ).toHaveCount(2);
+    );
 
     // Expect 2 indicators at the bottom
     await expect(frame.getByTestId("screenshare-indicator")).toHaveCount(2);

src/tile/MediaView.test.tsx

@@ -46,6 +46,21 @@ describe("MediaView", () => {
     });
   });
 
+  describe("with video", () => {
+    it("labels the video element with the track source", () => {
+      render(
+        <MediaView
+          {...baseProps}
+          video={mockVideoTrack({ source: "screenShare" })}
+        />,
+      );
+      expect(screen.getByTestId("video")).toHaveAttribute(
+        "data-source",
+        "screenShare",
+      );
+    });
+  });
+
   describe("name tag", () => {
     test("is shown with name", () => {
       render(<MediaView {...baseProps} displayName="Bob" />);

src/tile/MediaView.tsx

@@ -205,6 +205,7 @@ export const MediaView: FC<Props> = ({
             tabIndex={-1}
             disablePictureInPicture
             data-testid="video"
+            data-source={video.source}
             // Set the placeholder to a small transparent image. (On Android web
             // views the default poster image is particularly ugly.)
             poster={videoPlaceholder}