Properly set mode and NODE_ENV for test middleware

[eliperelman]

In the v8 line of Neutrino we manually default NODE_ENV to test when running the test command. With the removal of the Neutrino CLI, we would no longer have a test command, but we do still have testing middleware via the karma, mocha, and jest presets. We should ensure that we have a reasonable value for the config.mode and NODE_ENV for test middleware to rely on while trying to get the least number of config permutations.

At the outset, I'm wondering if we should have --mode none and NODE_ENV=test, but I think @edmorley had concerns about this particular mode. Also need to ensure that these values can be easily overridden from the host environment.

[edmorley]

I've been experimenting with setting mode to development when using Karma.

I've finally got it working, however I can see from the webpack build console output that the hot reloader configs are being added. This doesn't break anything, but is another side-effect of consolidating 9+ different permutations of configuration (command = {start, build, test, ...} x NODE_ENV = {production, development, test}) into just two (mode = {development, production}).

I'd be open to considering mode=none as a way to add a third state, though it would disable some of the webpack development features that might be useful for testing (eg named modules), so we'd want to vet those:
https://github.com/webpack/webpack/blob/v4.10.2/lib/WebpackOptionsDefaulter.js#L257-L266

Other possible options:

1. Make Neutrino support a mode of 'test', that gets translated to 'development' in the webpack config, but sets NODE_ENV to 'test'. Presets would then be encouraged to check the value of NODE_ENV rather than mode, so they would see all three states, not just the 2 stats of mode. (Though having Neutrino's mode support extra options is kinda confusing, so not sure this is ideal.)

2. Encourage presets to use NODE_ENV to check for development/production/test, and allow the adapters (neutrino().karma() etc) to set both a default mode and NODE_ENV.

[edmorley]

This is a blocker for releasing a v9 alpha. @eliperelman do you know when you might be able to take a look at this?

[eliperelman]

I'll try and start on this today.