Added removeCallback() test.

tests/testfut.nim

@@ -46,6 +46,30 @@ proc test3(): string =
   if fut.finished:
     result = testResult
 
+proc test4(): string =
+  var testResult = ""
+  var fut = testFuture1()
+  proc cb1(udata: pointer) =
+    testResult &= "1"
+  proc cb2(udata: pointer) =
+    testResult &= "2"
+  proc cb3(udata: pointer) =
+    testResult &= "3"
+  proc cb4(udata: pointer) =
+    testResult &= "4"
+  proc cb5(udata: pointer) =
+    testResult &= "5"
+  fut.addCallback cb1
+  fut.addCallback cb2
+  fut.addCallback cb3
+  fut.addCallback cb4
+  fut.addCallback cb5
+  fut.removeCallback cb3
+  discard waitFor(fut)
+  poll()
+  if fut.finished:
+    result = testResult
+
 when isMainModule:
   suite "Future[T] behavior test suite":
     test "Async undefined behavior (#7758) test":
@@ -54,3 +78,5 @@ when isMainModule:
       check test2() == true
     test "Future[T] callbacks are invoked in reverse order (#7197) test":
       check test3() == "12345"
+    test "Future[T] callbacks not changing order after removeCallback()":
+      check test4() == "1245"