Start tests for index exports (all currently fail)

[prescottprue]

Description

Started tests to confirm usage through require.

Using the library through require (as noted in docs and the examples) should be tested. It appears that all current tests only load src directly (never from top level index to confirm exports).

Code sample

Using like this should work:

// At the top of test/index.test.js
const test = require('firebase-functions-test')();
test.mockConfig({ some: { config: '' } });
test.wrap(myFunctions.makeUppercase);

[laurenzlong]

Great idea! Can you:

• make the tests more descriptive i.e. it('should export mockConfig as a function')
• add an after function which calls cleanup

[laurenzlong]

Thanks for the contribution!