Subzero maintains an internal scheduler that it uses only if there's no
external scheduler already bound on the current thread. This removes the
dependency on Marl in ReactorUnitTests, as we can rely on the internal
one being used.

Bug: b/145754674
Change-Id: Iddbaa299ccd904a87b8aa86e82e5517c9d72ef59
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41789Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>