fix scala#8956: add regression test

Author: bishabosha

tests/pos/i8956.scala

@@ -0,0 +1,22 @@
+type Numeric = Double | Int
+
+val v1 = 100
+val v2 = 100.0
+def check1(i: Double | Int | String): Unit = {
+  i match {
+    case a:(Double | Int) => println(s"numeric = $a")
+    case _ => println("categorical")
+  }
+}
+/*
+  [warn] 25 |      case a:Numeric => println(s"numeric = $a")
+  [warn]    |           ^^^^^^^^^
+  [warn]    |  the type test for gg.SlidingIssue.Numeric cannot be checked at runtime
+  [warn] one warning found
+  */
+def check2(i: Double | Int | String): Unit = {
+  i match {
+    case a:Numeric => println(s"numeric = $a")
+    case _ => println("categorical")
+  }
+}