Action cable testing

[palkan]

This PR is a remake of the older PRs (#23211 and #27191).

After discussing it again we decided to get it done for Rails 6.

This PR is the first one in the series (and the base). The idea is to merge action-cable-testing gem into Rails, step by step:

• Add Test adapter
• Add Test helper (for broadcasting tests)
• Add channels unit-testing support (Add ActionCable::Channel::TestCase #33969)
• Add connections unit-testing support (Add ActionCable::Connection::TestCase #34845)
• Generators (Add Action Cable testing guides and generators  #34933)
• Guides (Add Action Cable testing guides and generators  #34933)

/cc @dhh @matthewd @kaspth

[rails-bot]

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[dhh]

I like this! There are some improvements to the implementation we can make, but I'd be happy to merge this first, and then make those improvements. Thanks again 👌

[eileencodes]

Merged so we can make more progress and since DHH said he is happy to merge first.

[kaspth]

@palkan thanks for all your help with this! I'm sorry it took so long ❤️

[victorhazbun]

Thank you @palkan

[ahcarpenter]

Thanks for this @palkan!!

[rmacklin]

@palkan
After discovering anycable last year, I had found #23211 and had checked in on that PR from time to time. I'm super glad to see your contributions finally make it into rails after all this time! Thanks very much for persevering!

[scarroll32]

Until we all update to v6, there is this gem available.

[kunal6673]

Start