ZIO Test: Rename Predicate and Assertion

[adamgfraser]

It feels like #1438 is getting a little unwieldy so trying to break it up a bit. This resolves #1475 and changes Assertion to AssertResult and Predicate to Assertion along with various conforming changes.

The only thing I did not do is change FailureDetails to TestFailure since my understanding is that will stick around in the future in a slightly modified form as:

final case class FailureDetails(fragment: PredicateValue, whole: PredicateValue)
// to be refactored to be a nonempty list

sealed trait TestFailure[+E]
object TestFailure {
  final case class Assertion(result: AssertResult[FailureDetails]) extends TestFailure[Nothing]
  final case class Runtime[+E](cause: Cause[E]) extends TestFailure[E]
}

[jdegoes]

Great idea to break this up, & looks good to me!