Convert ioRuntime to lazy val

[ex0ns]

By making ioRuntime a lazy val in the AsyncIOSpec the order in which the trait are mixed in the trait does not matter as much as it is current.

Currently, by mixing AsyncIO trait BEFORE another trait/abstract class with a executionContext the execution fails with a Null Pointer Exception.
The following code shows this issue:

import cats.effect.testing.scalatest.AsyncIOSpec
import org.scalatest.flatspec.AsyncFlatSpecLike
import org.scalatest.matchers.should.Matchers
import cats.effect.IO

class Test extends AsyncFlatSpecLike with AsyncIOSpec with  Matchers {
  "A test" should "fail" in {
    IO { 1 shouldBe 2 }
  }
}
new Test().execute() // Test fails as expected

class TestFailure extends AsyncIOSpec with AsyncFlatSpecLike with  Matchers {
  "A test" should "fail" in {
    IO { 1 shouldBe 2 }
  }
}

new TestFailure().execute() // Fails with an NPE

I think that this change could prevent some mistake made when mixing multiple trait inside ScalaTest, I however don't have enough visibility on the impact caused by this change, and what would happen if multiple ExecutionContext were available when creating the ioRuntime.

Feel free to comment/reject/discuss. The workaround/fix is pretty easy to implement when using the library, but the NPE stacktrace is pretty confusing and not easy to debug when running the tests.

[djspiewak]

Honestly I think this is a good change and a nice catch!