feat: run parallel tests with different test contexts

I’d love to be able to initialise setupTest several times in the same file or even across files in the test suite.

E.g. do something like this

import { setupTest, createPage, cleanup } from '@nuxtjs/module-test-utils'

describe('module with default options', () => {
  setupTest({
    __dirname,
    browser: true,
    fixture: '../example',
  })

  test('My test', async () => {
    // ...
  })
})

describe('module with other options', () => {
  setupTest({
    __dirname,
    browser: true,
    fixture: '../example',
    config: differentOptions,
  })

  test('My test', async () => {
    // ...
  })
})

If I do this, I get non-determinative behaviour - or errors - even if the test is setup only once in each test spec file. (That is, options leak from one test context into another.) Or (if I pass different config objects in different files), I get:

 TypeError: Cannot read property 'moduleContainer' of undefined

   at node_modules/@nuxtjs/module-test-utils/dist/index.js:286:29
   at fulfilled (node_modules/tslib/tslib.js:112:62)

[pi0]

@danielroe #49 would be perfect if all internals can be instructed to use mock mfs. random build dir is also nice idea for a shorter term. Something else that would be awesome is if we can do snapshot builds similar to new nuxt generate to avoid unnecessary rebuilds when running tests multiple times. We may also introduce a lock-file to fix multi build conflict.

Also, issue with multiple describes in single file should be fixed by #51 (2.0.0-2) 😃

[pi0]

In fact, I’ve tested further and there is a conflict even when multiple files set up different contexts. The tests at least run but one context overwrites the other.

There is no shared context between multiple files no matter keeping context in test file or an imported library each test file is sandboxed in a worker. The reason multiple setup files are failing is mostly because the setup is also doing a nuxt build which because two runnin in parallel, conflicting on fs level (buildDir)

// test/shared.js
let state = 0

exports.inc = () => { state ++ }
exports.get = () => state

// test/a.test.js
const { get, inc } = require('./shared.js')

describe('A', () => {
  test('inc', () => {
    expect(get()).toBe(0)
    inc()
    expect(get()).toBe(1)
  })
})

// test/b.test.js
const { get, inc } = require('./shared.js')

describe('B', () => {
  test('inc', () => {
    expect(get()).toBe(0)
    inc()
    expect(get()).toBe(1)
  })
})