Fix: deprecation warning in RuleTester using Node v11

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Bug fix

Tell us about your environment

• ESLint Version:
• Node Version:
• npm Version:

What parser (default, Babel-ESLint, etc.) are you using?

Please show your full configuration:

Configuration

What did you do? Please include the actual source code causing the issue.

"use strict";
const RuleTester = require("eslint").RuleTester;
try {
    new RuleTester().run(
        "foo",
        context => (context.report(context.getSourceCode().ast, "message"), {}),
        { valid: [], invalid: [{ code: "", errors: [5] }] }
    );
} catch (err) {
    // ignore
}

What did you expect to happen?

I expected RuleTester to produce no output because the error was caught.

What actually happened? Please include the actual, raw output from ESLint.

RuleTester printed a deprecation warning:

(node:49648) [DEP0094] DeprecationWarning: assert.fail() with more than one argument is deprecated. Please use assert.strictEqual() instead or only pass a message.

What changes did you make? (Give an overview)

This updates RuleTester to avoid using the deprecated multi-argument assert.fail API in favor of the single-argument version. The single-argument version was introduced in Node v6.12.0, so we can use it since our support only covers Node ^6.14.0.

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

Nothing in particular