Added tests for stderr from missing tslint.json files

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: starts on Expand end-to-end tests for usage with CLI flags #89
• That issue was marked as status: accepting prs

Overview

Doesn't add all the tests I'd like to, but keeping PRs small and easy to read for now. Instead, this:

• Sets up tests to be one-group-per-test.ts file instead of auto-generating from test file names. The auto-generation is something I was holding on to from EightBittr unnecessarily.
• Adds a test for a missing tslint.json
• Fixes the test runner to catch non-0 exit code errors from exec as the result tested against