Re-encode relationships to avoid implicit conversion functions

[travisbrown]

This change only affects test code that I added recently, so it should be fairly uncontroversial.

In #3307 I introduced some abstraction for NonEmptyX testing, since there were a lot of inconsistencies and gaps not just in the interfaces themselves, but also in their tests.

In that change I encoded the relationship between the non-empty type and the type that has its operations as a couple of implicit parameters:

  ev: NE[Int] => NEC[Int],
  evPair: NE[(Int, Int)] => NEC[(Int, Int)]

I couldn't use <:< because for NonEmpty types that use the newtype encoding, NE isn't actually a subtype of NEC (e.g. NonEmptyChain and NonEmptyChainOps).

The issue is that this encoding in the tests doesn't work on Dotty, and in any case is a little inelegant—if for example we want to work with additional NE[x] values in the future, we'd need new evX parameters here.

I've changed NonEmptyCollectionSuite to use a new abstract method instead:

protected def toNonEmptyCollection[A](nea: NE[A]): NEC[A]

This fixes the tests on Dotty and provides a bit of extra future-proofing even on Scala 2.

[codecov-io]

Codecov Report

Merging #3323 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3323   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files         378      378           
  Lines        7684     7684           
  Branches      233      233           
=======================================
  Hits         7170     7170           
  Misses        514      514

Flag	Coverage Δ
#scala_version_212	93.38% <ø> (ø)	⬆️
#scala_version_213	93.09% <ø> (ø)	⬆️

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