frames/test/cypress/plugins/index.js

let server // static reference to the mock server, so we can close and re-assign on 2nd call

module.exports = (on, _config) => {
  on('task', {
    mockServer ({ interceptUrl, fixture }) {
      const fs = require('fs')
      const http = require('http')
      const { URL } = require('url')

      if (server) {
        // close any previous instance
        server.close()
      }

      const url = new URL(interceptUrl)
      server = http.createServer((req, res) => {
        if (req.url === url.pathname) {
          const data = fs.readFileSync(`./cypress/fixtures/${fixture}`)
          res.end(data)
        } else {
          res.end()
        }
      })

      server.listen(url.port)
      console.log(`listening at port ${url.port}`)

      return null
    }
  })
}