Update scalatest to 3.2.15

[scala-steward]

Updates org.scalatest:scalatest from 3.2.14 to 3.2.15.
GitHub Release Notes - Version Diff

I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.

If you'd like to skip this version, you can just close this PR. If you have any feedback, just mention me in the comments below.

Configure Scala Steward for your repository with a .scala-steward.conf file.

Have a fantastic day writing Scala!

Adjust future updates

Add this to your .scala-steward.conf file to ignore future updates of this dependency:

updates.ignore = [ { groupId = "org.scalatest", artifactId = "scalatest" } ]

Or, add this to slow down future updates of this dependency:

dependencyOverrides = [{
  pullRequests = { frequency = "@monthly" },
  dependency = { groupId = "org.scalatest", artifactId = "scalatest" }
}]

Note that the Scala Steward config file `.scala-steward.conf` wasn't parsed correctly

cron4s.InvalidCron: Either DayOfMonth and DayOfWeek must have a ? expression: DownField(frequency),DownField(pullRequests)

labels: library-update, early-semver-patch, semver-spec-patch, commit-count:1