fix missing .flatten method in TestFrameworkApi

Author: grzegorz-bielski

testkit/src/main/scala/com/evolutiongaming/catshelper/testkit/TestFrameworkApi.scala

@@ -1,6 +1,7 @@
 package com.evolutiongaming.catshelper.testkit
 
 import cats.effect.IO
+import cats.implicits._
 import cats.effect.testkit.TestContext
 import org.scalatest.exceptions.{TestCanceledException, TestFailedException}