added a basic ORC test I still had lying around (#15376)

tests/arc/torc_basic_test.nim

@@ -0,0 +1,138 @@
+discard """
+  output: "MEM 0"
+  cmd: "nim c --gc:orc $file"
+"""
+
+type
+  Node = ref object
+    name: char
+    sccId: int
+    #a: array[3, Node]
+    a0, a1, a2: Node
+    rc: int
+
+proc edge(a, b: Node) =
+  inc b.rc
+  if a.a0 == nil: a.a0 = b
+  elif a.a1 == nil: a.a1 = b
+  else: a.a2 = b
+  when false:
+    var i = 0
+    while a.a[i] != nil: inc i
+    a.a[i] = b
+
+proc createNode(name: char): Node =
+  new result
+  result.name = name
+
+#[
+
+     +--------------------------------+
+     v                                |
++---------+      +------+             |
+|         |      |      |             |
+|  A      +----->+      |      +------+------+
++--+------+      |      |      |             |
+   |             |      |      |     C       ------------>  G  <--|
+   |             |  R   |      |             |
++--v------+      |      |      +-------------+
+|         |      |      |        ^
+|   B     <------+      |        |
+|         |      |      +--------+
++---------+      |      |
+                 +------+
+
+]#
+proc use(x: Node) = discard
+
+proc main =
+  let a = createNode('A')
+  let b = createNode('B')
+  let r = createNode('R')
+  let c = createNode('C')
+
+  a.edge b
+  a.edge r
+
+  r.edge b
+  r.edge c
+
+  let g = createNode('G')
+  g.edge g
+  g.edge g
+
+  c.edge g
+  c.edge a
+
+  use g
+  use b
+
+proc buildComplexGraph: Node =
+  # see https://en.wikipedia.org/wiki/Strongly_connected_component for the
+  # graph:
+  let a = createNode('a')
+  let b = createNode('b')
+  let c = createNode('c')
+  let d = createNode('d')
+  let e = createNode('e')
+
+  a.edge c
+  c.edge b
+  c.edge e
+  b.edge a
+  d.edge c
+  e.edge d
+
+
+  let f = createNode('f')
+  b.edge f
+  e.edge f
+
+  let g = createNode('g')
+  let h = createNode('h')
+  let i = createNode('i')
+
+  f.edge g
+  f.edge i
+  g.edge h
+  h.edge i
+  i.edge g
+
+  let j = createNode('j')
+
+  h.edge j
+  i.edge j
+
+  let k = createNode('k')
+  let l = createNode('l')
+
+  f.edge k
+  k.edge l
+  l.edge k
+  k.edge j
+
+  let m = createNode('m')
+  let n = createNode('n')
+  let p = createNode('p')
+  let q = createNode('q')
+
+  m.edge n
+  n.edge p
+  n.edge q
+  q.edge p
+  p.edge m
+
+  q.edge k
+
+  d.edge m
+  e.edge n
+
+  result = a
+
+proc main2 =
+  let g = buildComplexGraph()
+
+main()
+main2()
+GC_fullCollect()
+echo "MEM ", getOccupiedMem()