Let the app define Testem middleware in testem.js rather than implicitly import middleware only from v1 addons

[lolmaus]

Rendered

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[ef4]

If we changed ember-cli to concatenate the addon-provided middlewares with whatever the app has put into testem.js, I think we can just call this a bug fix and it doesn't need an RFC.

It's also probably like a one-line change to the code.

[lolmaus]

@ef4 Yeah, but it offers less flexibility (can't opt out of v1 middleware), retains blackbox magic and does not push app maintainers towards Embroider future, where they will be supposed to configure their app by hand rather than relying on Ember CLI magic.

[ef4]

On my first pass I didn't notice that you proposed collectTestemMiddlewareFromV1Addons. That addresses my main concern, which is that if we simply disabled the classic behavior it would be very hard to adopt, since v1 addons never had to document what their middleware are and it's hard to discover.