Some Dotty compat for tests

[travisbrown]

Dotty doesn't like local implicit vals without type annotations. I'm finally getting back to Dotty cross-building for tests after doing this for core and laws a few weeks back. In my view these changes are an improvement even on Scala 2.

The only non-trivial change is in RepresentableSuite, where there was some kind of weird code like this:

implicit val bimonadInstance = Representable.bimonad[Pair, Boolean]

The issue here is that Representable.bimonad has a Representable[Pair] constraint, and the Representable[Pair] instance has a Functor[Pair] constraint. But bimonadInstance is a Functor[Pair]. If you leave off the type annotation Scala is fine with this (because Scala), but if you put it on you get forward reference errors from the compiler. I've worked around this by invoking the Representable[X] in an outer scope and assigning it to a non-implicit val.

I've also removed the single bracket in SemigroupKSuite because some other weird stuff was happening that seems entirely unrelated to what's being tested there.

[rossabaker]

👍 on green

[codecov-io]

Codecov Report

Merging #3253 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3253      +/-   ##
==========================================
+ Coverage   92.83%   93.05%   +0.22%     
==========================================
  Files         375      376       +1     
  Lines        7392     7415      +23     
  Branches      202      202              
==========================================
+ Hits         6862     6900      +38     
+ Misses        530      515      -15

Flag	Coverage Δ
#scala_version_212	93.38% <100%> (?)
#scala_version_213	92.83% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
...estkit/src/main/scala/cats/tests/ListWrapper.scala	100% <ø> (ø)	⬆️
...c/main/scala/cats/laws/UnorderedTraverseLaws.scala	78.94% <100%> (ø)	⬆️
laws/src/main/scala/cats/laws/TraverseLaws.scala	92.3% <100%> (ø)	⬆️
...rc/main/scala/cats/laws/NonEmptyTraverseLaws.scala	85% <100%> (ø)	⬆️
core/src/main/scala/cats/Show.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/instances/tuple.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Const.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Validated.scala	100% <0%> (ø)	⬆️
...cala-2.12/cats/kernel/compat/TraversableOnce.scala	0% <0%> (ø)
core/src/main/scala/cats/data/NonEmptyVector.scala	99.15% <0%> (+0.01%)	⬆️
... and 12 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0c8ca51...06608e9. Read the comment docs.