Validate that bigtest has test files to run against

[dagda1]

The bug that was raised for #800 has this testFiles glob:

 "testFiles": [  "src/**/*.test.{ts,js}"],

The correct directory is test and not src.

This is further complicated by the fact that the glob "testFiles": [ "src/**/*.test.{ts,js}"], actually found a match but it was a jest test.

import { waitFor } from './helpers';

describe('waitFor', () => {
 it('should wait for true', async () => {
   const fn = jest.fn().mockReturnValue(true);
// etc.

I definitely do not think this is a one-off and we should fail early if bigtest:

• finds no tests
• finds no bigtest tests

Descriptive console warnings should highlight what is wrong

[minkimcello]

Is this related to #635?