Removed test code from coro.nim and created three real tests for coroutines

tests/coroutines/titerators.nim

@@ -0,0 +1,26 @@
+import coro
+include system/timers
+
+var
+  stackCheckValue = 1100220033
+  numbers = newSeq[int](10)
+  i = 0
+
+iterator theIterator(id: int, sleep: float): int =
+  for i in 0..<5:
+    yield 10 * id + i
+    suspend(sleep)
+
+proc theCoroutine(id: int, sleep: float32) =
+  for n in theIterator(id, sleep):
+    numbers[i] = n
+    inc(i)
+
+var start = getTicks()
+start(proc() = theCoroutine(1, 0.01))
+start(proc() = theCoroutine(2, 0.011))
+run()
+var executionTime = getTicks() - start
+doAssert(executionTime >= 55_000_000.Nanos and executionTime < 56_000_000.Nanos, "Coroutines executed too short")
+doAssert(stackCheckValue == 1100220033, "Thread stack got corrupted")
+doAssert(numbers == @[10, 20, 11, 21, 12, 22, 13, 23, 14, 24], "Coroutines executed in incorrect order")