test: use drop instead of unreachable

This function is expected to trap, so the unreachable could result in a
false positive if the compiled test does not check the trap reason.

See comment:
#138 (comment)

Author: thibaudmichaud

test/core/stack-switching/cont.wast

@@ -127,7 +127,7 @@
     (cont.new $k1
       (ref.null $f1)
     )
-    (unreachable)
+    (drop)
   )
 )