bring back gl2d_line_select mock

... that was accidentally pushed to master in
    41c067a

    and removed in 1154589

Author: etpinard

test/image/mocks/gl2d_line_select.json

@@ -0,0 +1,14 @@
+{
+    "data": [{
+        "type": "scattergl",
+        "mode": "lines",
+        "x": [1, 2, 3],
+        "y": [1, 2, 1]
+    }],
+    "layout": {
+        "dragmode": "select",
+        "showlegend": false,
+        "width": 400,
+        "height": 400
+    }
+}