Add an ESLint rule to validate imports.

[toolness]

This addresses part of #1431 by adding a new ESLint rule to ensure that our production code never imports code from our test suite.

Here's an example of how it shows up in VSCode:

Note that in order to have this work in VSCode, the following must be added to one's .vscode/settings.json:

{
    "eslint.options": {
        "rulePaths": ["frontend/eslint/rules"]
    }
}

I'll mention this in our Setting up VSCode wiki page shortly after merging this.

[sraby]

Cool!