--test: allow passing path to tslint.json

[ajafff]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

This add the ability to pass the path to tslint.json instead of the directory containing it to tslint --test.
That's useful if you have a test directory structure like tslint and want to call it like tslint --test test/rules/**/tslint.json.

In fact this feature makes ruleTestRunner.ts obsolete. It could be simply replaced by the above command. Though that would run all tests before printing any output and would not stop at the first failed test.

Is there anything you'd like reviewers to focus on?

CHANGELOG.md entry: