Enable test forking

[djspiewak]

To avoid nondeterministic build failures like this one:

[info] cats.effect.internals.OnceOnlyTests *** ABORTED ***
[info]   java.util.ConcurrentModificationException: Two threads have apparently attempted to run a suite at the same time. This has resulted in both threads attempting to concurrently change the current Informer. Suite class name: cats.effect.internals.OnceOnlyTests
[info]   at org.scalatest.AsyncSuperEngine$$anonfun$runImpl$1.apply(AsyncEngine.scala:626)
[info]   at org.scalatest.AsyncSuperEngine$$anonfun$runImpl$1.apply(AsyncEngine.scala:623)
[info]   at org.scalatest.CompositeStatus.whenCompleted(Status.scala:886)
[info]   at org.scalatest.AsyncSuperEngine.runImpl(AsyncEngine.scala:623)
[info]   at org.scalatest.AsyncFunSuiteLike$class.run(AsyncFunSuiteLike.scala:216)
[info]   at org.scalatest.AsyncFunSuite.run(AsyncFunSuite.scala:2187)
[info]   at org.scalatest.tools.Framework.org$scalatest$tools$Framework$$runSuite(Framework.scala:314)
[info]   at org.scalatest.tools.Framework$ScalaTestTask.execute(Framework.scala:480)
[info]   at sbt.TestRunner.runTest$1(TestFramework.scala:76)
[info]   at sbt.TestRunner.run(TestFramework.scala:85)