Update scalatest to 3.2.15

[scala-steward]

Updates org.scalatest:scalatest from 3.2.4 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.

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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" } }] ```

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

[scala-steward]

Superseded by #216.