Use jsdom environment by default

[gaearon]

See my comment in #3881 (comment). Everyone just leaves this flag in.

We should either make it the default, or ship tests that don’t rely on jsdom by default.

[nickserv]

I disagree with changing the default, I think it should optimize for performance and boot up time by default, and many developers just need shallow rendering and won't realize JSDOM is being added unnecessarily. Though if it has to be changed can it please remain configurable?

[Timer]

See #5074

[Azure2020]

Yes

[Azure2020]

I want to start with you. What is required off me?