Decouple published modules from scalatest

[rossabaker]

This isolates our ScalaTest dependency to tests and docs so we can publish cats-2.0.0 before scalatest-3.1.0 is released.

• CatsSuite and CatsEquality, which depend on ScalaTest, are moved from cats-testkit to cats-tests' tests. This means we can continue to use them internally, but they are no longer available to external projects.

• The docs depend on discipline-scalatest for a full example. These examples assume a ScalaTest 3.1.0, which we hope will follow quickly after cats-2.0.0 is published. We can update the docs if there is a breaking change before scalatest-3.1.0.

If there is external demand for CatsSuite, we could consider a separate cats-scalatest repo with its own release cycle.

cats-testkit is no longer directly mentioned in the docs, calling into question its value as a module. That debate is for another PR, but is worth considering before we commit to cats-2.0.0.

[codecov-io]

Codecov Report

Merging #2970 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2970      +/-   ##
==========================================
+ Coverage   93.93%   93.98%   +0.04%     
==========================================
  Files         371      369       -2     
  Lines        7061     7052       -9     
  Branches      177      178       +1     
==========================================
- Hits         6633     6628       -5     
+ Misses        428      424       -4

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[kailuowang]

Nice! Thanks!

[djspiewak]

Random grammar fixes in the middle of an otherwise pure refactoring will never cease to bemuse and satisfy me.

[ceedubs]

I have no strong attachment to CatsSuite (the even method is evidence that it was never really intended for external usage). But CatsEquality is pretty handy for anyone who is using cats and Scalatest. It's small but finicky enough that it seems like it shouldn't be copied into every repo who wants its functionality. It might be useful to expose in a scalatest support module?

Now that I'm typing this out I'm realizing that I should probably just finally make the effort to convert some of my Scalatest tests to use a more lightweight testing library. But lots of people are still going to use Scalatest, and it's not easy for big code bases to transition all at once.

[rossabaker]

I wouldn't object to that being a microlibrary in a separate repo -- and we even could bring back a CatsSuite cleaned up for external use. But publishing that here would again put us and everything downstream at the mercy of ScalaTest's release cycle.

[ceedubs]

Ah sorry about that I didn't take the time to fully understand the context. I agree that it doesn't belong in this repo. And it is a small enough amount of code that I would probably copy and paste it as opposed to getting a new repo and build set up 😵

[kailuowang]

👍 for creating a Cats-testkit-scalatest repo. Copy pasting code from two files + finding out what extra dependencies are needed is a bit too much for someone trying to figure out how to law testing instances for cats type classes.