refactor: change default test directory name to "tests"

BREAKING CHANGE: all tests are now located in "tests" instead of "test"
close #877

Author: yyx990803

packages/@vue/cli-plugin-e2e-cypress/README.md

@@ -37,7 +37,7 @@ Cypress offers a rich interactive interface for running E2E tests, but currently
 
 ## Configuration
 
-We've pre-configured Cypress to place most of the e2e testing related files under `<projectRoot>/test/e2e`. You can also check out [how to configure Cypress via `cypress.json`](https://docs.cypress.io/guides/references/configuration.html#Options).
+We've pre-configured Cypress to place most of the e2e testing related files under `<projectRoot>/tests/e2e`. You can also check out [how to configure Cypress via `cypress.json`](https://docs.cypress.io/guides/references/configuration.html#Options).
 
 ## Installing in an Already Created Project
 

packages/@vue/cli-plugin-e2e-cypress/generator/template/cypress.json

@@ -1,3 +1,3 @@
 {
-  "pluginsFile": "test/e2e/plugins/index.js"
+  "pluginsFile": "tests/e2e/plugins/index.js"
 }

packages/@vue/cli-plugin-e2e-cypress/generator/template/test/e2e/plugins/index.js

@@ -0,0 +1,11 @@
+// https://docs.cypress.io/guides/guides/plugins-guide.html
+
+module.exports = (on, config) => {
+  return Object.assign({}, config, {
+    fixturesFolder: 'tests/e2e/fixtures',
+    integrationFolder: 'tests/e2e/specs',
+    screenshotsFolder: 'tests/e2e/screenshots',
+    videosFolder: 'tests/e2e/videos',
+    supportFile: 'tests/e2e/support/index.js'
+  })
+}