Fix #692: Move testcase to neg/

I don't think the `def run` in the original testcase should work given
that the added `def run2` doesn't in either scalac or dotty.

Author: smarter

tests/neg/t7868.scala

@@ -0,0 +1,13 @@
+object A {
+  def unapply(n: Int): Option[Int] = Some(n)
+
+  def run = (0: Short) match {
+    case A(_) => // error: this case is unreachable since class Short is not a subclass of class Int
+    case _    =>
+  }
+
+  def run2 = (0: Short) match {
+    case x: Int => // error: this case is unreachable since class Short is not a subclass of class Int
+    case _    =>
+  }
+}