update scalatest and discipline.

[kailuowang]

This shall enable a 2.0-M1 release

[kailuowang]

the property based test API in scalatest 3.1.0 has more breaking change than I thought. Closing this PR for now. will reopen once I fix them.

[codecov-io]

Codecov Report

Merging #2806 into master will decrease coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #2806     +/-   ##
=========================================
- Coverage   94.15%   93.85%   -0.3%     
=========================================
  Files         368      368             
  Lines        6914     6936     +22     
  Branches      296      298      +2     
=========================================
  Hits         6510     6510             
- Misses        404      426     +22

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/CatsSuite.scala	33.33% <ø> (ø)	⬆️

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[LukaJCB]

This looks good, though it worries me a bit that the ScalaTest Api has breaking changes in a minor version. I imagine this might affect people, using CatsSuite, no?

[kailuowang]

@LukaJCB I think in most use cases, it shouldn't affect users using CatsSuite. But part of the point of having this M1 release is so that we can test the impact. I can try it with Cats-tagless and see what migration is needed.