Add tests for examples from Nim in Action.

tests/niminaction/Chapter6/WikipediaStats/unguarded_access.nim

@@ -0,0 +1,15 @@
+import threadpool, locks
+
+var counterLock: Lock
+initLock(counterLock)
+var counter {.guard: counterLock.} = 0
+
+proc increment(x: int) =
+  for i in 0 .. <x:
+    let value = counter + 1
+    counter = value
+
+spawn increment(10_000)
+spawn increment(10_000)
+sync()
+echo(counter)