Update scalatest to 3.2.15

Author: 47erbot

build.sbt

@@ -26,7 +26,7 @@ lazy val exercises = (project in file("."))
       "joda-time"                   % "joda-time"                   % "2.12.2",
       "com.47deg"                  %% "scalacheck-toolbox-datetime" % "0.6.0",
       "com.chuusai"                %% "shapeless"                   % "2.3.10",
-      "org.scalatest"              %% "scalatest"                   % "3.2.14",
+      "org.scalatest"              %% "scalatest"                   % "3.2.15",
       "org.scalacheck"             %% "scalacheck"                  % "1.17.0",
       "org.scalatestplus"          %% "scalacheck-1-14"             % "3.2.2.0",
       "com.github.alexarchambault" %% "scalacheck-shapeless_1.15"   % "1.3.0"