Add syntax support for Scalatest "GivenWhenThen"

[AdamJKing]

A small wrapper for Scala-test's GivenWhenThen trait. This would help improve the ergonomics when writing BDD tests for effectful code. For example:

for {
  _ <- Resource.eval(IO(Given("given")))
  _ <- Resource.eval(IO(When("when")))
  _ <- Resource.eval(IO(Then("then")))
} yield ()

This is very noisy and makes reading BDD tests more difficult compared to...

for {
  _ <- Given("given")
  _ <- When("when")
  _ <- Then("then")
} yield ()

...where the intention is a lot clearer.

[armanbilge]

@typelevel/cats-effect-testing bump? :)