Fixes type returned by index function

[jonathan-beebe]

Description

I believe this fixes issue #7. I have been in conversation with the Firebase support team about this. Below is a super simple example of the issue I sent them.

Code sample

Before the following code would throw an error error TS2339: Property 'cleanup' does not exist on type 'FeaturesList'.

import * as functionsTest from 'firebase-functions-test'
const test = functionsTest()
test.cleanup()

Now with the new type TestFeatureList the cleanup function exists on the return type of functionsTest() and the code compiles and runs as expected.

[jonathan-beebe]

I am not super excited about the name TestFeatureList. I created it more as a proof of concept for the fix.

[laurenzlong]

Thanks for the proof of concept! I made #14 inspired by your PR. Closing this one in lieu of that one.