Add test setup for v3

[MonicaOlejniczak]

↪️ Pull Request

Adds V3 tests to CI via a new testing API that utilises a PARCEL_V3 environment variable to switch between which set of tests are to be run

Note that the parcelVersion logic may need to be refined later as needed

🤔 Considerations

Globals

I initially tried to override the globally defined describe and it rather than having to directly import the wrappers, but there are a couple of issues:

• when the globals are defined in another file such as the test utils, it either results in a circular reference or mocha is unable to attach the it handler due to its intertwined suite context
• the globals must be inlined in the test file itself, which even if generated will not have a good developer experience (lack of findable definitions, and slower performance due to code injection)

It also wasn't possible to use a root hook plugin or global fixture to inject the values.

💻 Examples

Run tests in both v2 and v3

describe('parcel', () => {
  it('bundles');
});

---

Run tests in v2 only

describe.v2('parcel', () => {
  it('bundles');
});

it.v2('test');

---

Run tests in v3 only

describe.v3('parcel', () => {
  it('bundles');
});

it.v3('test');

---

Run tests selectively in v2, v3, or both

describe('parcel', () => {
  it('bundles for both v2 and v3');

  it.v2('bundles for v2');

  it.v3('bundles for v3');
});

🚨 Test instructions

yarn test:integration

PARCEL_V3=false yarn test:integration

PARCEL_V3=true yarn test:integration