Specific commutativity and associativity tests for BigDecimal

[travisbrown]

Before the fix in #3303 we were seeing occasional failed tests on Scala 2.13, and we haven't seen any since then, but I should have added some tests that specifically highlighted the fix, to make sure the instance don't accidentally get changed back, etc.

In this PR I've added two specific cases where commutativity and associativity didn't hold for the old |+| for BigDecimal on Scala 2.13, which simply called the standard library's +:

import java.math.MathContext

val one = BigDecimal("1", MathContext.DECIMAL32)
val small = BigDecimal("0.00001111111", MathContext.DECIMAL32)
val xs = one :: List.fill(10)(small)

And then:

scala> xs.reduceLeft(_ + _)
res0: scala.math.BigDecimal = 1.000110

scala> xs.reduceRight(_ + _)
res1: scala.math.BigDecimal = 1.000111

And for commutativity:

scala> import java.math.MathContext
import java.math.MathContext

scala> val one = BigDecimal("1")
one: scala.math.BigDecimal = 1

scala> val small = BigDecimal("1e-7", MathContext.DECIMAL32)
small: scala.math.BigDecimal = 1E-7

scala> one + small
res0: scala.math.BigDecimal = 1.0000001

scala> small + one
res1: scala.math.BigDecimal = 1.000000

[codecov-io]

Codecov Report

Merging #3325 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3325   +/-   ##
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  Coverage   93.31%   93.31%           
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  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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[rossabaker]

Nice. I like this style of property tests and specific examples of things that have bitten us.