WIP - failing test for map overlay position

- need to build in a way to test line positions `template_test.go`
- run this test with `./hack/test-unit.sh -run TestYAMLTemplate
  TestYAMLTemplate.filetest=filetests/ytt-library/overlay/replace-key-position`

pkg/yamltemplate/filetests/ytt-library/overlay/replace-key-position.yml

@@ -0,0 +1,36 @@
+#@ load("@ytt:overlay", "overlay")
+#@ load("@ytt:template", "template")
+
+#@ def/end test1_left():
+---
+apiVersion: extensions/v1beta1
+kind: Ingress
+metadata:
+  name: example-ingress1
+  annotations:
+    key: 1
+
+#@ def/end test1_right():
+#@overlay/match by=overlay.all
+---
+metadata:
+  annotations:
+    key: 2
+
+---
+test1
+--- #@ template.replace(overlay.apply(test1_left(), test1_right()))
+---
++++
+
+ASSERT LINE NUM:
+
+          stdin:20 | [doc]
+                   |   test1
+           stdin:5 | [doc]
+           stdin:6 |   apiVersion: extensions/v1beta1
+           stdin:7 |   kind: Ingress
+          stdin:16 |   metadata:
+           stdin:9 |     name: example-ingress1
+          stdin:17 |     annotations:
+          stdin:18 |       key: 2