setupFilesAfterEnv and clearMocks support

[jehy]

This is a quick and a bit dirty solution for #24

It does not break anything but lets you use setupFilesAfterEnv and use clearMocks if it is enabled and exported from one of the setup files.

For example, we use sinon but we can enjoy independent mocks with a simple clearMocks implementation:

const sinon = require('sinon');
module.exports = {
	clearMocks: () => {
		sinon.sandbox.restore();
	}
};

I'm not sure if it will play as well with clearing jests's native mocks - but right now clearing native mocks does't work anyway.

[ljharb]

maybe only add the listener if clearMocks is set?

[jehy]

When creating reporter, we don't know if clearMocks exists yet.

We can move creating a reporter to after requiring setupFilesAfterEnv - but that's not like it is implemented in other runners, and I'm not sure if it will be correct.

[ljharb]

doesn't this mean that the last setup file to set clearMocks will wipe out any set by the previous files?

[jehy]

Defining clearMocks export it several files hardly makes sense... But we can add a check and throw error if clearMocks is redefined.

[ljharb]

Alternatively, could we build up an array of functions, and call them all in order?

I think it makes sense that anything you can do in one file, you can refactor to do in multiple.