test: add gatsby-node tests for plugin manifest (#8598)

* Add gatsby-node tests for plugin manifest

* Fixes after review

* Better fix the jest mocks

* Fix test to not fail for windows

packages/gatsby-plugin-manifest/src/__tests__/__snapshots__/gatsby-node.js.snap

@@ -0,0 +1,5 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Test plugin manifest options correctly works with default parameters 1`] = `"{\\"name\\":\\"GatsbyJS\\",\\"short_name\\":\\"GatsbyJS\\",\\"start_url\\":\\"/\\",\\"background_color\\":\\"#f7f0eb\\",\\"theme_color\\":\\"#a2466c\\",\\"display\\":\\"minimal-ui\\",\\"icons\\":[{\\"src\\":\\"icons/icon-48x48.png\\",\\"sizes\\":\\"48x48\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-72x72.png\\",\\"sizes\\":\\"72x72\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-96x96.png\\",\\"sizes\\":\\"96x96\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-144x144.png\\",\\"sizes\\":\\"144x144\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-192x192.png\\",\\"sizes\\":\\"192x192\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-256x256.png\\",\\"sizes\\":\\"256x256\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-384x384.png\\",\\"sizes\\":\\"384x384\\",\\"type\\":\\"image/png\\"},{\\"src\\":\\"icons/icon-512x512.png\\",\\"sizes\\":\\"512x512\\",\\"type\\":\\"image/png\\"}]}"`;
+
+exports[`Test plugin manifest options fails on non existing icon 1`] = `"icon (non/existing/path) does not exist as defined in gatsby-config.js. Make sure the file exists relative to the root of the site."`;

packages/gatsby-plugin-manifest/src/__tests__/gatsby-node.js

@@ -0,0 +1,47 @@
+jest.mock(`fs`, () => {
+  return {
+    existsSync: jest.fn().mockImplementation(() => true),
+    writeFileSync: jest.fn(),
+    statSync: jest.fn(),
+  }
+})
+const fs = require(`fs`)
+const path = require(`path`)
+const { onPostBootstrap } = require(`../gatsby-node`)
+
+describe(`Test plugin manifest options`, () => {
+  it(`correctly works with default parameters`, async () => {
+    await onPostBootstrap([], {
+      name: `GatsbyJS`,
+      short_name: `GatsbyJS`,
+      start_url: `/`,
+      background_color: `#f7f0eb`,
+      theme_color: `#a2466c`,
+      display: `minimal-ui`,
+    })
+    const [filePath, contents] = fs.writeFileSync.mock.calls[0]
+    expect(filePath).toEqual(path.join(`public`, `manifest.webmanifest`))
+    expect(contents).toMatchSnapshot()
+  })
+  it(`fails on non existing icon`, (done) => {
+    fs.statSync.mockReturnValueOnce({ isFile: () => false })
+    onPostBootstrap([], {
+      name: `GatsbyJS`,
+      short_name: `GatsbyJS`,
+      start_url: `/`,
+      background_color: `#f7f0eb`,
+      theme_color: `#a2466c`,
+      display: `minimal-ui`,
+      icon: `non/existing/path`,
+      icons: [{
+        src: `icons/icon-48x48.png`,
+        sizes: `48x48`,
+        type: `image/png`,
+      }],
+    })
+    .catch((err) => {
+      expect(err).toMatchSnapshot()
+      done()
+    })
+  })
+})