replace Stream in ExhaustiveCheck with List

[kailuowang]

Also added Scala version specific folders for tests
fixed a minor symbol package object file not in the package warning.

[codecov-io]

Codecov Report

Merging #2896 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2896      +/-   ##
=========================================
+ Coverage   94.17%   94.2%   +0.02%     
=========================================
  Files         368     368              
  Lines        6949    6949              
  Branches      304     309       +5     
=========================================
+ Hits         6544    6546       +2     
+ Misses        405     403       -2

Impacted Files	Coverage Δ
...rc/main/scala/cats/instances/SymbolInstances.scala	100% <ø> (ø)
.../src/main/scala/cats/laws/discipline/MiniInt.scala	97.29% <100%> (ø)	⬆️
...n/scala/cats/laws/discipline/ExhaustiveCheck.scala	100% <100%> (ø)	⬆️
...cala/cats/kernel/instances/FunctionInstances.scala	100% <0%> (+3.03%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	100% <0%> (+4%)	⬆️

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