Using --serve in test mode does not serve the test runner

[agubler]

Enhancement

At the moment using the --serve option when building in test mode is pretty useless as it serves the output/test directory meaning that it does not enable running the tests in the browser.

We should consider serving from the project root or disabling --serve when building the tests.

[mwistrand]

This is very similar to #38, which also throws in improved handling for the other modes.

[agubler]

@mwistrand excellent, I was transferring the issue from a comment on cli-build-webpack dojo/cli-build-webpack#259 (comment).

If it is a duplicate of #38 feel free to close.