add a bunch of tests for various plugins (gatsbyjs#1581)

* test: add test scaffolding for plugins

* test: add a simple test for navigateTo

* test: add a bunch of tests

* style: run prettier on tests

* test: fix broken absolute url

* test: fix equality check

* test: (hopefully) fix issue with windows

src/__tests__/gatsby-node.js

@@ -0,0 +1,3 @@
+const { sourceNodes } = require(`../gatsby-node`)
+
+test(`it has a test`, () => {})

src/__tests__/index.js

@@ -0,0 +1,20 @@
+const path = require(`path`)
+const { loadNodeContent } = require(`../`)
+
+describe(`gatsby-source-filesystem`, () => {
+  it(`can load the content of a file`, async () => {
+    const content = await loadNodeContent({
+      absolutePath: path.join(__dirname, `../index.js`),
+    })
+
+    expect(content.length).toBeGreaterThan(0)
+  })
+
+  it(`rejects if file not found`, async () => {
+    await loadNodeContent({
+      absolutePath: path.join(__dirname, `haha-not-a-real-file.js`),
+    }).catch(err => {
+      expect(err).toBeDefined()
+    })
+  })
+})