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

tests/coroutines/tgc.nim

@@ -0,0 +1,15 @@
+import coro
+
+var maxOccupiedMemory = 0
+
+proc testGC() =
+  var numbers = newSeq[int](100)
+  maxOccupiedMemory = max(maxOccupiedMemory, getOccupiedMem())
+  suspend(0)
+
+start(testGC)
+start(testGC)
+run()
+
+GC_fullCollect()
+doAssert(getOccupiedMem() < maxOccupiedMemory, "GC did not free any memory allocated in coroutines")