Add tests for examples from Nim in Action.

tests/niminaction/Chapter6/WikipediaStats/race_condition.nim

@@ -0,0 +1,13 @@
+import threadpool
+
+var counter = 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)