Begin stubbing out extend-node.js tests

Created a basic framework for creating a markdown node via the
onCreateNode function. This should be expanded to factor in the changes
that occur in the setFieldsOnGraphQLNodeType function.

packages/gatsby-transformer-remark/src/__tests__/extend-node.js

@@ -0,0 +1,100 @@
+const _ = require(`lodash`)
+const {
+    graphql,
+    GraphQLObjectType,
+    GraphQLList,
+    GraphQLSchema,
+  } = require(`graphql`)
+const { onCreateNode } = require(`../gatsby-node`)
+const { inferObjectStructureFromNodes } = require(`../../../gatsby/src/schema/infer-graphql-type`)
+
+// given a set of nodes and a query, return the result of the query
+async function queryResult(nodes, fragment, { types = [] } = {}) {
+    const schema = new GraphQLSchema({
+        query: new GraphQLObjectType({
+            name: `RootQueryType`,
+            fields: () => {
+                return {
+                    listNode: {
+                        name: `LISTNODE`,
+                        type: new GraphQLList(
+                            new GraphQLObjectType({
+                                name: `MarkdownRemark`,
+                                fields: inferObjectStructureFromNodes({
+                                    nodes,
+                                    types: [...types],
+                                }),
+                            })
+                        ),
+                        resolve() {
+                            return nodes
+                        },
+                    },
+                }
+            },
+        }),
+    })
+
+    const result = await graphql(
+        schema,
+        `query {
+            listNode {
+                ${fragment}
+            }
+          }
+        `,
+    )
+    return result
+}
+
+describe(`Excerpt is generated correctly from schema`, () => {
+
+    const node = {
+        id: `whatever`,
+        children: [],
+        internal: {
+          contentDigest: `whatever`,
+          mediaType: `text/markdown`,
+        },
+      }
+
+    // Make some fake functions its expecting.
+    const loadNodeContent = node => Promise.resolve(node.content)
+
+
+    it(`correctly loads a default excerpt`, async () => {
+        const content = `---
+title: "my little pony"
+date: "2017-09-18T23:19:51.246Z"
+---
+Where oh where is my little pony?
+`
+
+        node.content = content
+
+        const createNode = markdownNode => {
+            queryResult([ markdownNode ], 
+            `
+                excerpt
+                frontmatter {
+                    title
+                }
+            `,
+            {
+                types: [ { name: `MarkdownRemark` } ],
+            }).then(result => {
+                expect(_.isString(result.data.listNode.excerpt))
+            })
+        }
+        const createParentChildLink = jest.fn()
+        const boundActionCreators = { createNode, createParentChildLink }
+
+        await onCreateNode({
+            node,
+            loadNodeContent,
+            boundActionCreators,
+        })
+    })
+
+  })
+