added 2 GC leak test cases

tests/gc/stackrefleak.nim

@@ -0,0 +1,33 @@
+discard """
+  outputsub: "no leak: "
+"""
+
+type
+  Cyclic = object
+    sibling: PCyclic
+    data: array[0..200, char]
+
+  PCyclic = ref Cyclic
+
+proc makePair: PCyclic =
+  new(result)
+  new(result.sibling)
+  result.sibling.sibling = result
+
+proc loop =
+  for i in 0..10000:
+    var x = makePair()
+    GC_fullCollect()
+    x = nil
+    GC_fullCollect()
+
+  if getOccupiedMem() > 300_000:
+    echo "still a leak! ", getOccupiedMem()
+    quit(1)
+  else:
+    echo "no leak: ", getOccupiedMem()
+
+loop()
+
+
+