skyux test does not fail build

[Blackbaud-SteveBrush]

Expected behavior

skyux test should fail the build.

Actual behavior

The exit code is always 0. I believe it is related to webpack always being in watch mode: https://github.com/blackbaud/skyux-builder/blob/master/plugin/process-exit-code/index.js#L17

From the Webpack docs: "In webpack-dev-server and webpack-dev-middleware watch mode is enabled by default."

Steps to reproduce

1. create an obvious tslint error.
2. skyux test throws exit code 0