Need to validate config object returned from the plugins file

[bahmutov]

Cypress v3.6.1
Example in https://github.com/cypress-io/cypress-test-tiny/tree/return-invalid-value-from-plugins

When cypress.json has an invalid value we show a good error message during cypress open. For example, an invalid viewportWidth

{
  "viewportWidth": "foo"
}

leads to

and during cypress run shows the error

We found an invalid value in the file: `cypress.json`

Expected `viewportWidth` to be a number. Instead the value was: `"foo"`

But if we return invalid viewportWidth from file cypress/integration/plugins.js like this

module.exports = (on, config) => {
  // `on` is used to hook into various events Cypress emits
  // `config` is the resolved Cypress config
  return {
    viewportWidth: 'foo'
  }
}

The cypress open is happy, yet the browser shows NaN

and in this simple test cypress run finishes.

Using invalid baseUrl returned from plugins file breaks cypress run

module.exports = (on, config) => {
  // `on` is used to hook into various events Cypress emits
  // `config` is the resolved Cypress config
  return {
    viewportWidth: 'foo',
    baseUrl: 123
  }
}

$ npm run dev -- --run-project ~/git/cypress-test-tiny/

> [email protected] dev /Users/gleb/git/cypress
> node ./scripts/start.js "--run-project" "/Users/gleb/git/cypress-test-tiny/"

TypeError [ERR_INVALID_ARG_TYPE] [ERR_INVALID_ARG_TYPE]: The "url" argument must be of type string. Received type number
    at validateString (internal/validators.js:105:11)
    at Url.parse (url.js:154:3)
    at Object.urlParse [as parse] (url.js:149:13)
    at Object.exports.isListening (/Users/gleb/git/cypress/packages/server/lib/util/ensure-url.ts:50:18)
    at run (/Users/gleb/git/cypress/packages/server/lib/util/ensure-url.ts:29:24)
    at Object.exports.retryIsListening (/Users/gleb/git/cypress/packages/server/lib/util/ensure-url.ts:46:12)
    at Server._retryBaseUrlCheck (/Users/gleb/git/cypress/packages/server/lib/server.coffee:615:15)
    at /Users/gleb/git/cypress/packages/server/lib/server.coffee:246:23

Goal

Validate the configuration object returned from the plugins file the same way as we validate object loaded from cypress.json

[cypress-bot]

The code for this is done in cypress-io/cypress#5068, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[cypress-bot]

Released in 3.7.0.