Make preprocessors ignore node_modules/ by default or use nonegate minimatch option

Feature request - would it be possible to make `preprocessors` patterns ignore `node_modules/` by default? I have a repository where I'd like to be able to place tests (which need to be preprocessed by Webpack) anywhere, without providing an exhaustive list of the top-level directories they're in.

If I use a pattern like`'**/*.test.js': ['webpack']` in `preprocessors`, it's also picking up a `.test.js` file from deep within one of my dependencies in `node_modules/`, which has included tests in its npm package.

I can't use a leading `!node_modules/` because that will negate the whole pattern, as the `nonegate` minimatch option is not being used - perhaps enabling that option by default would be an easier solution? Since you can only provide one pattern per property in `preprocessors`, it doesn't make sense to pass in a negative pattern.