cli-plugin-unit-mocha test coverage support

[jorge-fernandes]

What problem does this feature solve?

Currently is next to impossible to use/configure vue-cli's cli-plugin-unit-mocha with nyc and get some reliable test coverage going. This feature would allow all devs to get out-of-the-box test coverage up and running.

As it stands now, it seems configuring webpack from scratch if easier than use vue-clie 3 out-of-the-box solutions, since everything is way easier and faster beside test coverage, which reverted me back to a pure webpack build.

If it's actually easy to accomplish this, perhaps just a up-to-date doc on how to actually achieve this can be a solution, but for now i can't get nyc to work in combination with vue-cli-service test:unit

What does the proposed API look like?

Perhaps cli-plugin-unit-mocha should also inject a new command for coverage:

vue-cli-service test:unit:coverage

[haoqunjiang]

Duplicate of #1363