feat(test:e2e): specify command to run for tests

[Ob-La-Di]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

vue-cli-service currently run a normal serve command for end to end test. This is an issue for people who use a custom command as explained here. This PR adds the possibility to pass a custom command to run instead of the default serve.

Example:

package.json

  "vuePlugins": {
    "service": [
      "vue-cli.test.js"
    ]
  }

vue-cli.test.js

const webpack = require('webpack');

module.exports = (api) => {
  api.registerCommand('serve:async', async (args) => {
    
    const result = await someAsyncFunction();

    api.chainWebpack((config) => {
      config.plugin('defineFoo').use(
        new webpack.DefinePlugin({
          FOO: JSON.stringify(result),
        }),
      );
    });

    // it's important to return the result here
    return api.service.run('serve', args);
  });
};

We need to define a global variable FOO to run build for production, but running vue-cli-service test:e2e fails because FOO will not be defined. This PR fixes the issue by running vue-cli-service test:e2e --command serve:async