test: Add a test for themes from dependencies using Layout.vue as entry point

Author: ocavue

__mocks__/vuepress-theme-without-index/Layout.vue

@@ -0,0 +1,2 @@
+module.exports = {}
+// This file is just for jest.

__mocks__/vuepress-theme-without-index/index.js

@@ -0,0 +1,4 @@
+{
+    "name": "vuepress-theme-without-index",
+    "main": "Layout.vue"
+}

__mocks__/vuepress-theme-without-index/package.json

@@ -18,6 +18,7 @@ describe('App', () => {
         theme: '@vuepress/default',
         emp: docsTempPath
       })
+      await app.process()
       expect(app.sourceDir).toBe(docsPath)
     }))
   })

packages/@vuepress/core/lib/node/__tests__/prepare/App.spec.js

@@ -0,0 +1,5 @@
+module.exports = {
+    title: 'Hello VuePress',
+    description: '# Hello, VuePress!',
+    theme: 'vuepress-theme-without-index'
+  }

packages/@vuepress/core/lib/node/__tests__/prepare/fixtures/docs-dep-theme-without-index/.vuepress/config.js

@@ -0,0 +1 @@
+# Hello, VuePress!