Add neg test for multiversal eq Nullable fix

Author: felixmulder

tests/neg/i1793.scala

@@ -0,0 +1,7 @@
+object Test {
+  import scala.ref.WeakReference
+  def unapply[T <: AnyVal](wr: WeakReference[T]): Option[T] = {
+    val x = wr.underlying.get
+    if (x != null) Some(x) else None // error
+  }
+}