Add test for consume error

Author: Linyxus

tests/neg-custom-args/captures/consume-twice.check

@@ -0,0 +1,13 @@
+-- Type Error: tests/neg-custom-args/captures/consume-twice.scala:6:7 --------------------------------------------------
+Separation failure: Illegal access to (x : Object^), which was passed to a
+consume parameter or was used as a prefix to a consume method
+and therefore is no longer available.
+
+where:    ^ refers to a fresh root capability in the type of parameter x
+5 |  send(x)
+  |       -
+  |       The capability was consumed here.
+6 |  send(x)  // error
+  |       ^
+  |       Then, it was used here
+

tests/neg-custom-args/captures/consume-twice.scala

@@ -0,0 +1,6 @@
+import language.experimental.captureChecking
+import language.experimental.separationChecking
+def send(consume x: Object^): Unit = ()
+def consumeTwice(consume x: Object^): Unit =
+  send(x)
+  send(x)  // error