gc:destructors: add first test program

tests/destructor/tgcdestructors.nim

@@ -0,0 +1,17 @@
+discard """
+  cmd: '''nim c --gc:destructors $file'''
+  output: '''1 1'''
+"""
+
+import allocators
+include system / ansi_c
+
+proc main =
+  var s: seq[string] = @[]
+  for i in 0..<80: s.add "foo"
+
+main()
+
+#echo s
+let (a, d) = allocCounters()
+cprintf("%ld %ld\n", a, d)