Implement ScalaTest ioRetrying with a generic effect type

[bcarter97]

In order to use the ioRetrying defined below, we need a concrete IO. I think it would better for this to take a generic effect type (even though it would use IO in the base trait), so that helper traits don't need to be tied to IO.

cats-effect-testing/scalatest/jvm/src/main/scala/cats/effect/testing/scalatest/AsyncIOSpec.scala

Line 31 in 0dbc4c3

implicit def ioRetrying[T]: Retrying[IO[T]] = new Retrying[IO[T]] {