Mention TestScope.backgroundScope in runTest timeout messages

The new message is added when the test body finishes in time,
but the child coroutines don't.

Fixes #4182

Author: dkhalanskyjb

kotlinx-coroutines-test/common/src/TestBuilders.kt

@@ -340,9 +340,13 @@ public fun TestScope.runTest(
                         var message = "After waiting for $timeout"
                         if (completionCause == null)
                             message += ", the test coroutine is not completing"
-                        if (activeChildren.isNotEmpty())
+                        if (activeChildren.isNotEmpty()) {
                             message += ", there were active child jobs: $activeChildren"
-                        if (completionCause != null && activeChildren.isEmpty()) {
+                            if (completionCause != null) {
+                                message += ". Use `TestScope.backgroundScope` " +
+                                    "to launch the coroutines that need to be cancelled when the test body finishes"
+                            }
+                        } else if (completionCause != null) {
                             message += if (scope.isCompleted)
                                 ", the test coroutine completed"
                             else