added test cases from strange loop event

tests/showoff/tonce.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''first call of p
+some call of p
+new instantiation
+some call of p'''
+"""
+
+template once(body: stmt) =
+  var x {.global.} = false
+  if not x:
+    x = true
+    body
+
+proc p() =
+  once:
+    echo "first call of p"
+  echo "some call of p"
+
+p()
+once:
+  echo "new instantiation"
+p()