Add walking skeleton pipeline fixture

internal/framework/pipeline/testdata/walking_skeleton_input.json

@@ -0,0 +1,17 @@
+{
+  "id": "fixture-source",
+  "units": [
+    {
+      "id": "u1",
+      "text": "First event."
+    },
+    {
+      "id": "u2",
+      "text": "Second event."
+    },
+    {
+      "id": "u3",
+      "text": "Third event."
+    }
+  ]
+}